LPHT annealing of brown-to-yellow type Ia diamonds

Accepted Manuscript LPHT annealing of brown-to-yellow type Ia diamonds

Sally Eaton-Magaña, Troy Ardon, Alexander M. Zaitsev PII: DOI: Reference:

S0925-9635(17)30212-1 doi: 10.1016/j.diamond.2017.06.008 DIAMAT 6895

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

29 April 2017 27 May 2017 15 June 2017

Please cite this article as: Sally Eaton-Magaña, Troy Ardon, Alexander M. Zaitsev , LPHT annealing of brown-to-yellow type Ia diamonds, Diamond & Related Materials (2017), doi: 10.1016/j.diamond.2017.06.008

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ACCEPTED MANUSCRIPT LPHT annealing of brown-to-yellow type Ia diamonds Sally Eaton-Magaña,1 Troy Ardon,1 Alexander M. Zaitsev2 Gemological Institute of America, Carlsbad, California, USA 2 College of Staten Island, Staten Island, New York, USA

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Low-pressure, high-temperature (LPHT) annealing of yellow-to-brown type Ia natural diamonds was performed to monitor its effects on optical centers within diamond, changes in the observed color, and to assess the process's viability as a commercial gem treatment. With LPHT annealing only, the mostly brown diamonds showed a shift towards yellow coloration; Vis-NIR absorption spectra showed this change was due to a modest increase in H3 intensity. Even at long annealing times (24 hours at 1800oC) or annealing at high temperatures (2000oC for five minutes), the diamonds did not significantly lose brown coloration. LPHT annealing showed itself as an ineffective means to break apart the vacancy clusters causing the brown color or causing nitrogen disaggregation, which resulted in only a small H3 generation With LPHT annealing, “amber centers” — a group of several independent bands in the IR between 4200 and 4000 cm-1 that disappear with HPHT annealing — were seen to anneal out gradually at various temperatures from 1700–2000oC. In contrast, high-pressure, high-temperature (HPHT) annealing effectively removes brown color at similar time/temperature conditions. Without the high stabilizing pressure provided by HPHT annealing techniques, the LPHT annealing showed pronounced damage on inclusions and dramatic surface etching. In subsequent experiments, LPHT annealing was used as a follow-up to laboratory irradiation. The irradiation-related vacancies created greater concentrations of H3 and the vacancy-assisted disaggregation of nitrogen created donors which led to a high concentration of H2 centers. This combination of defects resulted in a pronounced and favorable shift towards saleable yellow colors due to an increase in H3 and a dramatic increase in the H2 center, which led to the suppression of the remaining brownish component. The annealing characteristics for many centers detected by Vis-NIR absorption spectroscopy, FTIR absorption spectroscopy, and photoluminescence spectroscopy were chronicled throughout the study and compared with other LPHT annealing studies and HPHT annealing experiments. 1. Introduction Natural diamonds can occur as almost any color, but yellow-to-brown is the most common. The brown color is attributed to vacancy clusters [1,2] for the vast majority of natural diamonds, though there are several other possible origins [3]. The transformation of structural defects in brown diamonds has principally focused on samples subjected to high temperature, high pressure annealing in the diamond stability region (i.e., HPHT treatment) [4,5]. A few studies of LPHT annealing on natural [5,6] and synthetic [5,7,8] diamonds have been reported. 1

ACCEPTED MANUSCRIPT This study focuses on type Ia diamonds as these have the largest natural abundance (~98%). Therefore, a comparatively inexpensive treatment such as LPHT annealing, if it proved successful, could be used extensively among brown-to-yellow type Ia diamonds for the gem market. Type Ia diamonds containing A and B nitrogen centers generally become yellow to green-yellow after HPHT annealing due to isolated nitrogen and/or the significant absorption by H3 and H2 centers [4,5].

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HPHT treatment occurs at high pressures (5–7 GPa) and at high temperatures and approximates the conditions to which natural diamonds were subjected while deep in the earth’s mantle. Within HPHT annealing there are several different temperature regimes: [3] 1600–1900°C: Rarely used but can remove brown color when performed for several hours. 1900–2100°C: Most defects causing brown color anneal out. Major nitrogen-vacancy related defects are formed. 2100–2300°C: brown color removed in several minutes. Single nitrogen defects formed in type Ia diamond. Most commercial treatments occur within this temperature regime.

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The necessary technology and expense to achieve high pressure annealing can be prohibitive and therefore a low-pressure option would prove advantageous if it resulted in a commercially viable treatment. The temperatures (1700–2000oC) and pressures (< 1 atm) used in these annealing experiments are within the graphite stability zone, where diamond transforms into graphite. This transformation does not happen instantaneously in the bulk diamond at these conditions. Instead, crystal surfaces and fracture surfaces are initially affected, resulting in etching and pitting, as well as graphitization. Therefore, LPHT annealing of diamond for the gem market is a competition between noticeably changing the color of the diamond and the concurrent damage of etching, surface graphitization, and internal graphitization at the diamond/inclusion interfaces. Such resulting damage can overwhelm any gains in value made by the color treatment. This article explores the spectroscopic and visual changes arising from this treatment and evaluates the tradeoffs between 1 “C” (color) and the remaining three “C”’s (cut, carat, and clarity).

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1.1 Optical centers As numerous optical centers are affected throughout the annealing range; they will be briefly summarized here. For additional information and example spectra, several reference sources are available (e.g., ref. [3] and ref. [9] and references therein). In FTIR absorption, the A aggregates (1280 cm-1) are a pair of nitrogen atoms and the B aggregates (1170 cm-1) are the combination of four nitrogen atoms with a vacancy. Single substitutional nitrogen (C defect) shows an IR peak at 1344 cm-1. Platelets are a feature generally detected at 1360–1380 cm-1. While the configuration is currently unknown, the accepted model is a {001} planar configuration of interstitial carbon atoms around a B aggregate [10]. The 3107 cm-1 peak is an important spectroscopic feature in diamond with the accepted model as the stretch mode of VN3H defect [11] while the 1405 cm-1 center is ascribed as the bending vibrational mode [11]. The H1a (1450 cm-1), H1b (4940 cm-1), and H1c (5170 cm-1) centers are reliably observed in diamonds that have been irradiated and annealed at temperatures between 500-600oC and 1400-1500oC. While the models are not conclusive, the H1a center contains nitrogen

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2. Materials and Methods

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interstitials [12,13], the H1b correlates with a defect including A aggregates, and the H1c defect correlates with a defect including B aggregates [5]. Amber centers are a general term describing at least four peaks (most commonly 4165, 4113, 4073, and 4065 cm-1) that are seen in natural brown diamonds. The name derives from the initial observation that this series of features occurs in “amber”-colored (i.e., brown) natural diamonds [14]. This name is given to several different bands predominantly in the range of 4500-4000 cm-1 (though the full series of amber centers is described to extend from 9000-3900 cm-1 [14]) with the most common absorption band given as the feature at 4160 cm-1 [14,15] and tentatively ascribed as a defective A-aggregate [14]. These centers are destroyed by commercial HPHT treatments [15,16]. In photoluminescence (PL) and UV-Vis-NIR absorption spectroscopy, the H3 center (zero-phonon line [ZPL] at 503.2 nm) is the A aggregate with an added vacancy and the H4 center (ZPL = 495.9 nm) is a B aggregate with an added vacancy. The H2 (ZPL at 986.2 nm) is the negatively charged H3 center (i.e., [NVN]-). The N3 center (ZPL at 415.2 nm) is a trio of nitrogen atoms bonded with a vacancy. The GR1 (ZPL at 741.2 nm) is the neutral vacancy (V0). In UV-Vis-NIR absorption, the 595 nm center is considered a radiation-related feature, generally observed only in strongly irradiated diamonds. In PL spectroscopy, there are several additional centers detected in these diamonds. The NV0 (ZPL at 575 nm) is the neutral nitrogen-vacancy center and NV- (ZPL at 637 nm) is the negatively charged nitrogen-vacancy center. The 490.7 nm center is a naturally occurring feature, generally associated with plastic deformation and hypothesized as a nitrogen-vacancy complex. The 490.7, 535.8, 558.2, 575.9, and 612.4 nm peaks along with a triplet feature at ~566 nm are commonly observed in PL spectra of natural diamonds and many were affected by heating within the temperature range used in this study. The N3, H3, H4, and H2 centers may also be seen in the UV-Vis-NIR absorption spectra of naturally-sourced type Ia diamonds.

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For this investigation, 12 brown-to-yellow type Ia natural faceted diamonds were purchased from Lotus Color (table 1). Additionally, 14 other natural rough diamonds were laser cut into thin plates and annealed along with the faceted diamonds in Part I. These other samples were intended to chronicle the effect of LPHT annealing on specific spectroscopic features and some individual inclusions, but the majority of the data and assessment are based on the results of the faceted diamonds. 2.1. Part I: LPHT Annealing Only These natural diamonds were divided into two different tracks for step-wise annealing: 1) longer time annealing and 2) higher temperature annealing. The changes in appearance, FTIR absorption, UV-Vis-NIR absorption spectroscopy, and photoluminescence (PL) spectroscopy were assessed after each annealing step. Longer Time Annealing  1700oC, 1 hour  1700oC, 24 hours  1800oC, 1 hour 3

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 1800oC, 24 hours  1900oC, 10 minutes Higher Temperature Annealing  1700oC, 1 hour  1800oC, 1 hour  1900oC, 15 minutes  1900oC, 1 hour  2000oC, 3 minutes

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2.2. Part II: Irradiation/LPHT annealing After Part I was complete, the 11 surviving faceted samples were subsequently subjected to an additional sequence of experiments. The diamonds were similarly documented after each treatment step. 1) E-beam irradiation using a dose of 2.9×1018 e/cm2 at 1 MeV (termed as “lower dose” later in article). 2) LPHT annealing at 1700oC for 1 hour 3) E-beam irradiation using a dose of 1.0×1019 e/cm2 at 3 MeV (termed as “higher dose” later in article). 4) Low temperature annealing at 1000oC for 10 minutes (two samples, LC1 and LC2, were not annealed at this condition) 5) LPHT annealing at 1900oC for 5 minutes

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The Vis-NIR spectra were collected at liquid nitrogen temperature using a custom-built instrument attached with Avantes AvaSpec-2048 spectrometer (4 channels), and two broad-band light sources (AvaLight-HAL and AvaLight-DH-S). This spectrometer’s range is 250–1000 nm, but light source limitations restrict its usage in most circumstances to ~375 to 900 nm. Entrance slit width for each spectrometer of the four channels is 10 microns. Two hundred scans were collected for each sample in order to achieve a good signal-to-noise ratio within this wavelength range. This spectrometer has high resolution to detect the optical defects present within a diamond, but is not suitable for quantitative assessment; therefore these measurements cannot be used to determine defect concentrations, such as the H3 center [17]. After each annealing step, the diamonds were color graded by specially trained personnel according to grading standards (e.g., ref. [18]). The colors of the diamonds were also assessed using a custom-made imaging device and then plotted within CIE L (Lightness) C (Chroma) H (Hue) color space. The measured hue angle can vary from values of 0o to 359o and the chroma values were scaled against the values obtained for the starting material. Additionally, clarity grading was performed by GIA using standard diamond grading nomenclature. Images of the diamonds' internal features were collected with a Nikon DXM1200F Ultrahigh-definition digital camera attached to SMZ1500 photomicroscope or a Nikon Eclipse LV100. This microscope utilizes a focus stacking technique that combines 50 images or more at different focal depths. This method provided an extended depth of field using Nikon Elements D software. To gain an understanding of the impurities present in the samples, Fourier-transform infrared (FTIR) absorption spectra covering the 400–6000 cm–1 range were taken using a Thermo Nicolet Nexus 6700 spectrometer furnished with KBr and quartz beam splitters and a diffuse-reflectance infrared Fourier transform (DRIFT) accessory. Nitrogen A and B aggregate 4

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and single substitutional nitrogen concentrations were determined using a customized computer algorithm derived from a spreadsheet provided by Dr. David Fisher (DTC Research Center, Maidenhead) [19-22]. Photoluminescence (PL) spectra with 325, 457, 488, 514, 633, and 830 nm laser excitation using a Renishaw inVia Raman microscope at liquid nitrogen temperature were collected on all samples. As PL spectra are semi-quantitative, the peak areas determined from the spectra were normalized using the ratio of the peak area to the calculated area of the diamond Raman peak in each instance. Using GRAMS software, the spectral peaks observed within the Vis-NIR absorption spectra and PL spectra were fit to a mixed Gaussian/Lorentzian curve. For each peak in each sample after each annealing step, the fitting determined the peak intensity, full width at halfmaximum, and peak areas above a software-determined baseline.

2.3. LPHT Annealing Furnace

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The LPHT treatment was conducted using an experimental homemade system described previously [7,8]. The samples were placed in the graphite heating tube which was injected with flowing hydrogen gas at 2 mL/second and a pressure of 0.5 atm. The furnace could be ramped up to the target temperature within minutes. The central portion of the furnace was a hollow graphite heater tube of 10 mm in size with an attached W-Re thermocouple. The calibration of the thermocouple was accomplished using the melting points of pure Al, Cu, Au, Si, Pt and Rh and any errors of temperature measurement were lower than 10oC. To avoid overheating, the device was cooled with running water. The flowing hydrogen gas significantly reduced the degree of graphitization compared to annealing in vacuum or with an inert gas such as argon. However, surface graphitization was often observed after each annealing step, the diamonds were placed in oxygen plasma for 30 minutes and then in boiling sulfuric acid for one hour.

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3.1. Visual appearance In Part I, most of the 12 faceted diamonds demonstrated significant weight loss due to graphitization and etching with the annealing. Several plates and 2 faceted diamonds broke apart (LC5 and LC12). Typically after HPHT annealing, faceted diamonds are repolished due to surficial frosting; however with LPHT annealing, there was also significant weight loss. While there is minimal improvement in color grade in many of the diamonds during Part I, it does not appear to offset the damage wrought on the carat weight and clarity (table 1). The diamonds subjected to Longer Time Annealing seemed to fare better in terms of resulting weight loss and decrease in clarity grades. These diamonds lost an average of 15% of their weight due to LPHT annealing and another 13% due to repolishing. Clarity was reduced by 1–2 grades. However, there were only minor changes in the color grades. After the repolishing following the 1800oC, 24 hour annealing, all six diamonds in this group still retained a brown color. One diamond (LC9; Fancy brown-yellow) showed no change in color grade. Two other

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diamonds (LC10 and LC11) showed no change in hue (yellowish brown and brown, respectively), and only a change in tone (Fancy Dark to Fancy). The diamonds subjected to Higher Temperature Annealing showed significant degradation in weight and clarity. One diamond (LC5) broke apart into several pieces following the 1900oC, 1 hour annealing and considered a total loss in terms of this experiment. Another sample during that annealing broke into two pieces (LC12) although the larger fragment was considered salvageable for additional testing. Due to the more extreme conditions than those within the Longer Time Anneal, these diamonds lost an average of 30% of their weight due to LPHT annealing and another 13% due to repolishing (LC5 not included in averages). Among the five surviving diamonds, the clarity grades were reduced by 2–3 grades. The dominant color is no longer brown (e.g., LC6, LC8, LC12), but still present as a modifier. The two diamonds which had an orange modifier as natural shifted towards yellow hues. The diamonds with the lightest color initially (LC1–LC4) all ended with a color grade of Fancy brownish yellow. As the tone of the light-colored diamonds tended towards a “Fancy” color, so too did the three Fancy Dark diamonds (LC10, LC11, LC12) as all three ended with a “Fancy” color as well. Many of the diamonds were asymmetrically etched and the repolished diamonds show significant flaws in the outline shape and are off-round (e.g., LC9 from the Longer Time Anneal and LC3, LC6, LC8, and LC12 from the Higher Temperature Anneal). In Part II, following the laboratory irradiation, the diamonds appeared green in color due to the high concentration of the GR1 defect. Visual and spectroscopic changes due to irradiation have been studied in detail elsewhere [23,24] and the results at this intermediate step will not be expanded upon here. After the low-dose irradiation/LPHT annealing, the diamonds showed a remarkable improvement over Part I results. The combination of treatments changed the diamonds to saturated colors with yellow hues (table 1). Four diamonds had an especially desirable color grade of Fancy Intense yellow. The type IaB diamond (LC7) retained brown as the dominant color (graded as fancy yellowish brown) and a type IaA<
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Figure. 1. “■----” Initially light brown diamonds; “▲─ - ─” Initially moderate (“fancy”) brown diamonds “♦─” Initially dark brown diamonds. Blue plots indicate Longer Time Annealing during Part I. Red plots indicate Higher Temperature Annealing during Part I.

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3.2. Inclusions Internal graphitization does not occur within perfect diamond at these temperatures [25], but requires the presence of a defect such as microcracks [26] or other inclusions of other minerals, e.g. olivine [27]. As expected, some of the internal inclusions showed pronounced graphitization and some internal crystals developed fractures. In a few cases, the fractures were large enough to split the samples into pieces (e.g., LC5, LC12). Several of the diamonds showed etching patterns on the surface in addition to etch pits. The thermal etching is essentially a reversal of crystal growth [28], which can be seen as the hexagonal frosted patterns on the surface of the sample (figure 2a). This internal damage (e.g., figure 2b and 2c) from extended LPHT annealing lowers the clarity grades for these diamonds as it enlarges minor inclusions and the darkening caused by internal graphitization makes the defects far more obvious thus reducing the value of these diamonds (figure 3). In some cases, the resulting damage led to a total loss of the diamond. 7

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Figure 2.(a) Thin-plate 469-ABB shows hexagonal etching and graphitized etch pits. This image was collected after annealing to 2000°C for three minutes. (b) Thin-plate 919-B at the unheated and 1800oC, (c) 1 hour annealing stages. The elongated crystal developed a large stress halo, but the other two did not. This is most likely due to the elongated crystal lying along a grain plane (indicated by the brown graining) where the other two are not directly on a grain plane. Image widths: 1.5 mm.

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Fig 3. After the first annealing step (1700oC, 1 hour), LC5 showed pronounced graphitization of internal inclusions (the other samples showed similar graphitization, but to a smaller degree. This sample broke apart during the 1900 oC, 15 minute annealing step.

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3.3. IR Absorption Spectra

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These diamonds were all type Ia with a wide variety of relative aggregation (ratio of A to B centers) and total nitrogen concentration (table 1). The IR absorption intensity of the main A and B nitrogen centers (at 1280 and 1170 cm-1, respectively) was not observed to change through the annealing steps of Part I (table 2). The A aggregate showed a decrease and the B aggregate showed little change following the high-dose irradiation/LPHT annealing stages. The integrated intensity of the platelet peak at 1360–1370 cm-1 showed little to no change across the annealing experiments and the spectral position did not show a shift as well. However, its peak width did show a slight broadening with the heating (average of 30% increase in FWHM from the unheated diamonds to the final stage). The hydrogen-related peak (ascribed as N3VH [11] ) at 3107 cm-1 in the IR spectra was present and showed a slight decrease across the annealing experiments for a majority of the diamonds (LC1, LC3, LC4, LC7, LC8, LC12); the cumulative decrease across all annealing experiments was by a factor of 2-3. In the other samples, the 3107 cm-1 peak was no longer detected after annealing at 1900oC. The associated peak at 1405 cm-1 disappeared from most samples following the 1900oC annealing. In the samples in which the 1405 cm-1 peak remained (LC7, LC8, LC12), it was detected throughout the subsequent annealing experiments. In Part II, the H1a, H1b, and H1c peaks were prominent following the high-dose irradiation and annealing at 1000oC. The three peaks developed in nearly all diamonds. The type IaB diamond, LC7, showed only the H1c. Unsurprisingly, the absorption of H1b correlated with the A aggregate concentration and the H1c intensity correlated with the B aggregates. The peaks annealed out after the subsequent heating at 1900oC. Also after the 1000oC annealing, peaks developed at 2677 cm-1 (LC6,LC8,LC11), 2917 -1 cm (LC3, LC4, LC6, LC7, LC8, LC12) and at 4434 cm-1 (LC3, LC3, LC8, LC9). After 1900oC annealing, the 2677 cm-1 feature was no longer detected and the 2917 cm-1 center became slightly larger. Also, the feature at 4434 cm-1 was no longer detected, but a prominent band at 4396 cm-1 appeared in all annealed diamonds. These features (previously named as H1d, e, f, g) are ascribed as electron transitions [29]. A small peak associated with single substitutional nitrogen center at 1344 cm-1 developed after the high-dose irradiation and 1900oC annealing in 8 of the 11 annealed diamonds. The concentration of calculated C defects ranged from 5-15 ppm. All of the unheated diamonds in this suite showed the amber center at 4165 cm-1. A few (LC2, LC3; figure 4b) also showed the center at 4065 cm-1; additionally a few had bands at 4210 and 4355 cm-1. In contrast, a survey of some natural type Ia pink diamonds showed many of those samples contained the amber center at 4065 cm-1 and only one contained the 4165 cm-1 center [30]. Previous studies have shown that commercial HPHT treatment at temperatures greater than 2000oC destroys the amber centers [15,16]. Therefore, when gem diamonds have been evaluated for possible heat treatment, the presence of any of the amber centers generally was interpreted as evidence that no heat treatment had occurred. Additionally, while assessing a diamond's treatment history, there has not been special attention given as to which amber center 9

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was detected within a natural diamond. Typically, the presence of any amber center was deemed sufficient to establish a natural diamond had not been subjected to heat treatment for color modification. To our knowledge, there have not been studies chronicling the step-wise destruction of the amber centers (e.g., figure 4a). In a few samples (LC2, LC3), both the amber centers at 4165 and 4065 cm-1 were present in the natural diamond (figure 4b). After 1700oC annealing for one hour, the features at 4355, 4210, and 4065 cm-1 are gone while the 4165 cm-1 defect still survives (figure 4c). The 4165 cm-1 center noticeably decreased following the 1900oC annealing and this region of the mid-IR absorption spectra is essentially featureless after a brief annealing at 2000oC, which is consistent with HPHT annealing observations.

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Figure 4. (a) Change in mid-IR absorption features for LC3 as the natural sample is heated from 1700 oC to 2000oC. The spectra are adjusted vertically for clarity. (b) The “amber center” region of the mid-IR absorption spectra for the natural samples show several features associated with natural diamonds (including bands at 4355, 4210, 4165, and 4065 cm -1. The two spectra at top are the diamonds (LC2, LC3) that also showed the 4065 cm-1 peak. (c) After the first annealing step (1700oC for 1 hour), several amber centers were no longer observed including those at 4355 and 4210 cm-1, and the most notable change, the amber center at 4065 cm-1. FilFilee##66: :LC3-IR-MAY LC3-IR-MAY LC3 LC3

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3.4. Vis-NIR Absorption Spectra Due to the method of data collection, these Vis-NIR spectra, collected at liquid nitrogen temperature, are semi-quantitative as there is an unknown path length. However the ratios of absorbance peaks detected within the same spectrum may be calculated and those ratios may be compared with ratios calculated from other spectra. As N3 concentration is stable up to temperatures of 1900oC or greater [3], this center provided the best possible internal normalization and the data collected during Part I of the experimentation are provided in figure 5, left. Throughout this temperature range, the average value of H3 (normalized to the N3 center) increases as the diamonds are annealed at progressively higher temperatures and times. This increase in the H3 center causes the shift of the diamond color towards yellow (figure 5, right). However, the brown absorption continuum that is present within the natural diamond is still observed throughout the Part I experiments and preserves the brownish component in the diamonds’ color grades. A summary of the progression of various centers is shown in table 2.

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Figure 5. (a) The peak area ratio (H3/N3) collected by Vis-NIR absorption spectroscopy increased as the heated

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samples were stepwise annealed up to 1900-2000oC. As the collected data are semi-quantitative, the peak areas within each spectrum were normalized to the N3absorption center. The normalized intensity data were then averaged for the 11 faceted samples. Error bars show one standard deviation. (b) Individual Vis-NIR absorption spectra for LC8 at several annealing stages. At right, for clarity, the spectra are displaced and not all spectra are shown.

In Part I, as the samples were annealed from 1700–2000oC, the increase in H3 is the most obvious change in the Vis-NIR absorption spectra. The 550 nm band reduced in several samples, but still remained (figure 5b). This reduction in the 550 nm band is in contrast to prior observations seen with HPHT annealing that causes a strengthening of the so-called Pink Band [3]. However, the 550 nm band could no longer be observed following the irradiation and annealing steps. A weak 480 nm band is observed is most of the diamonds prior to heating. The peak was still detected following the initial annealing at 1700oC for one hour. It is difficult to gauge its

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continued presence at higher annealing temperatures as the feature is obscured by the sideband associated with the increasing H3 center. Immediately following irradiation, the diamonds’ observed color is green and the VisNIR absorption spectra show the expected features (not shown) including a pronounced GR1 peak at 741.2 nm along with its accompanying sideband and the 595 nm center associated with irradiation treatment. With low-dose irradiation/LPHT annealing, the Vis-NIR absorption spectra show the continued increase in the H3 center and introduction of a modest H2. The H2/H3 absorption at this stage were sufficient to create a yellow color. The changes with high-dose irradiation/LPHT annealing are even more dramatic. Most significantly though is the pronounced increase of the H2 center. This center’s (ZPL at 986.2 nm) sideband extends well into the visible range. The combination of the H2 and H3 centers creates a transmission window within the green-yellow part of the spectrum and helps generate the vibrant colors seen following the high-dose irradiation/LPHT annealing steps.

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3.5. Photoluminescence Spectroscopy

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Annealing at these high temperatures caused several changes in the resulting photoluminescence spectra (figure 6). PL spectroscopy is a very sensitive analytical technique that can detect optical defects at much lower concentrations than absorption spectroscopy. Therefore, the PL method (where spectral features are excited by incident laser light of specific wavelengths) is capable of detecting optical centers in diamond when Vis-NIR absorption cannot. A summary of the progression of various centers is shown in Table 2.

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3.5.1. Features that decreased/were eliminated There are several optical centers whose configuration is unknown, but their appearance in PL spectra generally correlates with natural diamonds (i.e., 490.7, 558.2, 612.4, and 814 nm). These centers were eliminated by heating at these temperatures. The 612.4 nm feature was in seven of the faceted natural diamonds and destroyed in all seven with the initial heating at 1700oC for 1 hour. The peak at 558.2 nm (LC2, LC3, LC4) survived the initial annealing at 1700oC, but was eliminated with subsequent annealing at 1700-1800oC. The series at 566 nm was maintained through 1800oC. The peak at 814 nm (LC2, LC9) was eliminated with annealing at 1800-1900oC. The intensity of the 490.7 nm peak (detected in all 12) decreased, but was still detected throughout Part I of these experiments and temperatures up to 2000oC (figure 7d). Prior results suggested that the peak anneals out completely at 1700-1800oC in HPHT-treated diamond [3]. The thermal stability of this peak in these samples is in marked contrast with some naturally irradiated type Ia green diamonds in which the 490.7 nm peak annealed out after heating to 800oC [24]. The H4 center was detected in 8 of the 12 faceted diamonds and was gone after annealing at 1800-1900oC, comparable with prior results [5]. Diamonds with the 480 nm band feature in Vis-NIR absorption spectra also show a distinctive series of PL peaks in the 600–640 nm range [31,32]. While the 480 nm band could not be reliably detected in the Vis-NIR absorption spectra after annealing, the associated PL feature was seen in most diamonds throughout Part I. This series of peaks disappeared from most diamonds after the high-dose irradiation/1000oC annealing and its intensity decreased in the samples in which the series was still detected (LC1, LC3, LC6, LC8). 13

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1800oC, 1 hr

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GR1

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Natural

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3.5.2. Features that increased/were introduced As expected, H2 and H3 showed marked increases in intensity within this temperature range (figures 7a and 7b). Although, the H2 center showed a noticeable decrease after the first annealing step, it increased in subsequent heating experiments. Within the Part I experiments, the H2 center had increased slightly, most within a factor of 2. This increase was considered quite small especially when compared with increases in later experiments. In Part II, the H2 center showed a dramatic increase after the post-irradiation annealing of 2–3 orders of magnitude. The H3 center increased about one order of magnitude within Part I experiments and another factor of 10 during the Part II experiments. Considering that H3 is an intermediate step in H2 generation, the continued increase of H3 (along with H2) indicates that a significant amount of H3 is generated in Part II. With Part II (combination of irradiation/LPHT annealing), a few features that showed little change throughout Part I showed noticeable increases. The PL spectra for the N3 center showed consistent values throughout much of the annealing and showed a slight increase with the final annealing; consistent with prior results [4]. Additionally, a few centers of unknown configuration at 535.8 and 575.9 nm showed little change or slight increases throughout Part I and showed more pronounced increases during the irradiation/LPHT annealing stage (e.g., figure 7c). The NV0 and NV- centers (ZPLs at 575 and 637 nm, respectively) were introduced in Part II after the low-dose irradiation/LPHT annealing step. The NV centers showed a maximum intensity following the subsequent irradiation and annealing at the lower temperature of 1000oC and decreased after the LPHT annealing at 1900oC (figures 7e and 7f); this temporary increase at an intermediate temperature in NV centers was seen previously for comparable LPHT annealing experiments with CVD-grown diamonds [33]. The ratio of negative-to-neutral NV centers for each sample increased with subsequent annealing thus showing, along with the introduction of the 1344 cm-1 peak and the increase of H2 centers, that irradiation/LPHT annealing of type Ia diamonds causes the dissociation of A centers and the increase in nitrogen donors (C defects).

Diamond Raman

1900oC, 1 hr -50000

20000

490

495

500

2000oC, 5 min

-100000

2000oC, 3 min

0

505

510

515

520

525

550

Wavelength (nm) Counts / Nanometers

600

650

700

750

Wavelength (nm) Ov erlay Y -Zoom SCR OLLCounts / Nanometers

Ov erlay Y -Zoom CUR SOR

Figure 6. PL spectra were collected after each treatment/annealing step on all diamonds. (a) The 488 nm PL spectra for sample LC8 shows the increase in H3 at 503.2 nm. (b) The 514 nm PL spectra for sample LC2 shows the elimination of several features at 558.2 and 612.4 nm and the GR1 at 741.2 nm along with the continued presence of more stable features such as the optical centers at 535.8 and 575.9 nm. For 488 nm excitation, the diamond Raman peak is located at 521.9 nm. For 514 nm excitation, the diamond Raman peak is located at 552.4 nm. For clarity of presentation, not all spectra collected after each annealing step are shown and the spectra are overlaid, but displaced vertically. For adequate comparison of the individual spectra, the diamond Raman peaks are scaled as equivalent.

File # 5 = LC8-2000C -3MIN-488B

Yrsa 488 PL (492-530): LC 8-2000C-3min

4/21/2011 10:52 PM Res=N one File # 3 = LC2-1800C -1HR-514A

4/21/2011 10:49 PM Res=N one

Yrsa 514 PL (517-850): sm, lc2-1800C-1hr

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1000

1000 (e)

(f)

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"High dose"/LPHT

"High dose" /Low temp.

"Low dose"/LPHT

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2000oC, 3 min

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Figure 7. These plots show the peak areas (normalized to the diamond Raman peak) for the (a) H2 center (using 830 nm excitation), (b) H3 center (using 488 nm excitation), (c) 575.9 nm peak (using 514 nm excitation), (d) 490.7 nm peak (using 457 nm excitation), (e) NV0 center (using 514 nm excitation), and (f) NV- center (using 514 nm excitation). “■----” Initially light

brown diamonds; “▲─ - ─” Initially moderate brown diamonds; “♦─” Initially dark brown diamonds. Blue plots indicate Longer Time Annealing during Part I. Red plots indicate Higher Temperature Annealing during Part I.

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ACCEPTED MANUSCRIPT 4. Discussion 4.1. Comparison of defect centers with prior LPHT and HPHT annealing experiments

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The results seen here are consistent with prior LPHT experiments on type Ia diamond [5]. Here, we observe an increase in H2 and H3 and evidence of nitrogen disaggregation such as the marked increase in H2, the slight decrease in A aggregates, the detection of C centers at 1344 cm-1 during the final stage of annealing, and a greater ratio of NV- to NV0 centers. In contrast, LPHT experiments on low-nitrogen type Ib material show opposing trends [8] and evidence of nitrogen aggregation with a decrease in H2 centers, decrease in C centers, and a greater ratio of NV0 to NV- centers. Some prior LPHT experiments on natural diamonds [6] consisted of 5-6 pulses of thermal shock up to high temperature and showed a considerable lightening of dark brown type Ia diamonds; they also reported no generation of H2 centers in PL spectroscopy. They suggested that the thermal gradients created by their method aided in annihilating the dislocations which assisted in reducing the brown color. The experiments performed here were conducted with continuous heating at temperature, comparable to the continuous temperature profile experienced by diamonds subjected to HPHT annealing. LPHT annealing alone that was performed during Part I did not achieve significant lightening of the brown color and it is likely that the thermal pulses of ref. [6] were more effective in that regard than the constant temperature LPHT annealing of the Part I experiments performed here. In comparison with prior LPHT annealing experiments, there were differences in thermal stability for various centers, but the data are qualitatively comparable. Here the H4 center annealed out after 1900oC annealing; in prior studies, it annealed out at 1600oC [5]. Additionally, the 490.7 nm center showed high thermal stability in this study. Although it decreased, it survived heating to 2000oC and remained through most of the Part II experiments. In other LPHT annealing studies, the center is almost eliminated at 1600oC [5] or anneals out entirely [6]. In HPHT treatment of diamonds, platelet dissociation has been observed at temperatures as low as 1800oC [3] and the associated peak in IR absorption was also seen to decrease using pulsed LPHT annealing [6]. However, in these experiments, there was little change in the integrated intensity of the platelet absorption peak. HPHT annealing also broadens the platelet peak [34] and can shift the position to lower wavenumbers [10]. While this suite of diamonds showed a modest broadening of the platelet peak, it did not show a decrease in intensity or a shift in position. HPHT annealing is usually performed for very short time periods at higher temperatures than what was performed here. The higher temperature leads to dissociation of aggregates and creation of C defects. The LPHT annealing performed here occurred at temperatures several hundred degrees lower than the 2300oC heating typically used for commercial HPHT annealing. While lower pressure favors dissociation of A aggregates [35], no detectable C centers were produced and minimal indirect evidence of disaggregation (e.g., H2 centers) during Part I due to the comparatively lower temperatures used. Longer times could have been used as well, but would have exacerbated the surface graphitization effects of not heating at a stabilizing pressure. As such, we conclude that a commercial treatment of LPHT annealing alone would not be feasible.

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The amber centers at 4165 and 4065 cm-1 (among others) in these natural diamonds showed a reduction, distinct thermal stabilities, and ultimately elimination with LPHT annealing. The 4065 cm-1 annealed out after the initial heating at 1700oC for one hour while the 4165 cm-1 center survived several heating cycles. The conditions that cause brown color in natural type Ia diamonds also produces the amber centers and HPHT annealing is shown to eliminate the vacancy clusters that cause brown coloration along with the amber centers. However, the LPHT annealing experiments here showed that the amber centers were destroyed while the brown coloration was not. Although the formation conditions of the two features are similar, the thermal stability of amber centers is lower than that of the vacancy clusters. Additionally, some type Ib CVD diamonds that showed peaks at similar positions and were tentatively ascribed as amber centers. In contrast, the amber centers in the CVD diamonds survived LPHT annealing [8], thus indicating that the thermal stability of amber centers are affected either by the growth method or the diamond type. 4.2. Color change with annealing: H3/H2 defects

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In HPHT-annealed type Ia diamonds, the final product is often a yellow-green color due to the creation of H3/H2 defects and the removal of brown color [4,5]. With HPHT annealing this defect formation pathway is typically achieved by breaking apart vacancy clusters and creating single substitutional nitrogen (C centers). The H3 concentration is increased by the addition of these vacancies to an A center and the H2 concentration is increased when a created H3 center combines with an electron generated by the formation of the C center. With LPHT annealing alone in Part I, we see some increase in the H3 concentration and minimal change in the H2 concentration. There is little detectable dissociation of A aggregates and therefore low concentrations of C centers. Also, from the minor decrease in brown coloration, there is also a low amount of dissociation of the vacancy clusters. The low amount of available vacancies causes only a small increase in H3. Concurrently, with the absence to nearabsence of C centers, H3 centers do not convert their charge and H2 is not effectively produced. However, the combination of irradiation/LPHT annealing far better creates H3 and H2 defects and this series of treatments produces outcomes that mimic colors created by HPHT annealing. Therefore, while the reaction pathway is different between HPHT annealing and irradiation/LPHT annealing, the resulting products are comparable. With the irradiation in Part II, this treatment creates a much higher concentration of accessible vacancies (i.e., not trapped within vacancy clusters and that are independent of vacancies contributing to brown color). These vacancies can then act to create H3 and a percentage of these will transform to H2 when it finds a donor electron from a C center. The C centers are more efficiently created in the LPHT annealing following the irradiation as the dissociation of A aggregates is aided by the available vacancies. The dissociation of nitrogen aggregates when assisted by vacancies has been documented numerous times [36]. While dissociation of nitrogen aggregates has been observed at low temperatures of 800oC [36], the 1000oC annealing did not create sufficient absorption of H2/H3, and did not create appealing colors. The Vis-NIR spectra after 1000oC annealing were dominated by the remaining GR1 and the 595 nm center. With subsequent annealing at 1900oC, the irradiation-related centers were no longer detected and only H2/H3 contributed to the diamonds' color.

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ACCEPTED MANUSCRIPT Other researchers [36] have demonstrated that irradiation and low-temperature annealing at different conditions than those used here, either in terms of irradiation dose and annealing time can effectively produce H3/H4 defects and change the color to a commercially viable yellow instead of the brown coloration observed in these experiments following the “high-dose” irradiation/1000oC annealing.

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4.3. Coloration from Brown Continuum As the brown color in natural diamond is derived from vacancy clusters [2], a higher saturation of brown color potentially indicates a greater source of vacancies and would result in a greater change in H3 concentration after annealing. While this trend has been observed in HPHT annealing [4], it was not duplicated here thus providing additional verification that the LPHT annealing does not appreciably affect the vacancy clusters and the resulting brown coloration. The brownish component appears still present throughout Part I and in the type IaB diamond throughout Part II. The time/temperature regime used in these LPHT experiments was within the range in which vacancy clusters should be broken apart, but this does not seem to occur to a very noticeable degree (figure 8). The low pressure used here does not break up the vacancy clusters as effectively as with HPHT annealing. This result is not surprising as dissociation of vacancies, along with small radius impurity atoms, is aided by high-pressure treatments instead of low pressure [3]. It appears that brown color wasn’t removed so much as it was suppressed by the addition of yellow-to-green color through H3 generation in Part I and H3/H2 centers created in Part II. Prior research has shown that the brown color in CVD synthetic diamond is effectively removed by LPHT annealing [8] although this result is not surprising as the defect related to brown color in CVD diamonds has a lower thermal stability than their natural analogues. CVD synthetic diamonds do not undergo significant plastic deformation and the brown color is annealed out at lower temperatures than in natural diamonds [38]. In CVD diamond, the origin of the brown color has not been conclusively demonstrated and both vacancy clusters and nondiamond carbon are possible causes [38-40].

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Figure 8. The annealing time required to achieve an order of magnitude change in concentration of defects for activation energies from 6 to 10 eV. The black circles indicate the time/temperature conditions for each annealing experiment in Part I. Although the annealing steps were conducted within a time-temperature regime that should bring about significant dissociation of A aggregates and dissociation of vacancy clusters, there was little observation of these reactions. A aggregation did not proceed to a noticeable degree until assisted by electron irradiation. After Dobrinets et al., 2013 [3], figure 9.2.

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4.3. Commercial viability of treatment on gem diamonds

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The treatment of irradiation/LPHT annealing could find a niche within the commercial gem trade, but it would likely be applied almost exclusively to rough diamonds instead of polished goods and to diamonds pre-selected based on high clarity (a concession already applied to HPHT annealing). The very modest improvement in color due to LPHT annealing alone likely signifies that this treatment method would only be advantageous when coupled with irradiation as potentially a more cost-effective method of treatment than HPHT annealing. The reduction of the 550 nm band indicates LPHT annealing would be unlikely for treatment of brown-to-pink diamonds. Comparison of the color grades for these samples against the relative concentrations of A and B aggregates in table I indicates that diamonds of type IaA=B or Type IaA>B develop more marketable colors. Additionally, the presence of A aggregates appears to be the most important parameter in determining the color grades after LPHT annealing in Part I or the irradiation/LPHT annealing combination in Part II. The saturation of the brown color in the starting material seems to have little effect on the color grades of the treated diamonds. The intensity (in terms of magnitude or percentage change) of the major defect centers detected by Vis-NIR absorption and PL spectroscopy did not appear to be substantively influenced by the extent of brown coloration in the starting material. As evidenced by the modest changes in the type IaB diamond, LC7, these treatments likely do little to affect the brown color component within the diamonds. The change of color is

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5. Conclusion

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that creates H3 and H2 defects. At high concentration, these centers create sufficient absorption to overwhelm the underlying brownish component. The saturation of the green color in Part II did not appear to correspond with either the starting brown saturation or the A aggregate concentration. With continued annealing, the greenish color among the type IaAB diamonds would likely increase as the H2 centers developed further. With LPHT annealing alone, the generation of H3 is insufficient to suppress the brownish color component in these diamonds and the long times (or high temperatures) needed to affect this color change creates collateral damage in terms of graphitization and inclusion alterations. If treaters choose to pursue LPHT annealing for changing the color of brown natural type Ia material, we anticipate that the combination of irradiation followed by short LPHT annealing (less than 10 minutes) on type IaA=B or type IaA>B material would be the most successful. It would bring about a desirable color change (a “low-dose” irradiation would likely result in yellow hues while a “high-dose” irradiation would correspond to green-yellow hues). The short LPHT annealing time necessary for this combination of treatments would minimize the damaging effects on surface graphitization and on internal inclusions that were seen during the longer times of LPHT annealing in Part I. Pure type IaB material is not advisable for this treatment since the mechanism requires H3/H2 generation in order to achieve color change—as such, a measureable quantity of A aggregates is desirable. The pure type IaB (LC7) diamond showed the lowest change in hue angle and the lowest intensity of H2. As LPHT annealing does not effectively remove brown color, type IaB diamonds are not assisted by this treatment. With appropriate selection of diamond rough, the irradiation/LPHT annealing combination of treatments could prove useful within the gem diamond market.

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With LPHT annealing alone, the original brown color of the samples was slightly reduced and the yellow color was increased, but the majority of the diamonds still retained a brownish color modifier or brown color. While there were modest improvements in diamond color, the LPHT treatment caused a significantly negative impact of two other “C’s”: carat and clarity. When LPHT annealing is used in conjunction with irradiation, the irradiation-derived vacancies create sufficient H3 and precipitate adequate nitrogen disaggregation to dramatically increase the concentration of H3 and H2 centers. With this increase comes a color change in the type IaA±B to significantly alter the color, depending on irradiation conditions, to yellow or yellow-green. Several optical centers detected by photoluminescence spectroscopy were chronicled for their thermal stability. The 490.7 nm center survived to very high temperatures in these samples

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ACCEPTED MANUSCRIPT and the distinct annealing characteristics for individual amber centers was chronicled for the first time.

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ACKNOWLEDGMENTS: We are grateful to Wuyi Wang for procuring the samples. Ulrika D’Haenens-Johansson, Jonathan Muyal, and Kyaw Soe Moe are thanked for their assistance with data collection. Ivan Tsang and Melissa Santos assisted with the color and clarity grading. One of the authors (AMZ) acknowledges the support of this research by PSC-CUNY Research Foundation (Grant 68082-00 46).

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[40] R. Jones, "Dislocations, vacancies, and the brown colour of CVD and natural diamond," Diam. Relat. Mater., 18 (2009) 820.

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Sample number

Color

A aggregates/ B aggregates (ppm)

After "Lower Dose" Irradiation / LPHT Annealing

--- After LPHT Annealing / Subsequent Repolishing ---

-------------------Unheated--------------------

Clarity Weight (ct)

Longer Time Annealing

Color

Weight loss(ct) (% loss Etching / % loss repolishing)

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Color

After "Higher Dose" Irradiation / 1000 oC Annealing

After "Higher Dose" Irradiation / LPHT Annealing; Subsequent Repolishing

Color

Color

---After 1800°C, 24 hr annealing---

LC1

S-T,Light brown

114/208

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0.31

Fancy brownish yellow

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Y-Z, Light brown

70/96

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0.42

Fancy brownish yellow

SI2

-0.09 ct (-12%/-10%)

Fancy intense yellow

LC7

Fancy orangebrown

0/133

VS1

0.32

Fancy brownyellow

SI1

-0.15 ct (-28%/-13%)

Fancy yellowish brown

LC9

Fancy brownyellow

66/0

VS1

0.25

Fancy brownyellow

VS2

-0.10 ct (-24%/-16%)

Fancy yellow

LC10

Fancy Dark yellowish brown

16/26

SI1

0.30

Fancy yellowish brown

I1

-0.07 ct (-10%/-13%)

LC11

Fancy Dark brown

303/125

SI1

0.32

Fancy Light brown

SI2

-0.05 ct (-6%/16%)

Higher Temperature Annealing -------------------Unheated-------------------LC2

Y-Z, Light brown

19/5

SI1

0.30

LC3

Y-Z, Light brown

138/194

SI1

0.35

LC5

Y-Z, Light brown

4/146

SI1

LC6

Fancy yellowbrown

306/132

VS1

LC8

Fancy orangy brown

426/364

LC12

Fancy Dark orangebrown

52/205

-0.06 ct (-10%/-10%)

Fancy yellow

T P

D E

Fancy dark yellowish brown

I R

T P

Fancy deep yellowish green

Final weight (ct) / Total % loss

0.10, 0.09 / Not repolished

Fancy deep greenyellow

I1

0.24 / 42%

Fancy brownyellow

SI1

0.10 / 68%

Fancy dark yellowish brown

Fancy deep greenish yellow

SI1

0.08 / 68%

Fancy greenish yellow

Fancy dark yellowish brown

Fancy deep greenish yellow

SI1

0.14 / 54%

Fancy yellow

Fancy dark yellow-brown

Fancy deep brownish greenish yellow

U N

A

M

C S Fancy dark yellowish brown

0.19 / Not repolished

---After 1900°C, 1 hr annealing--Fancy brownish yellow

I1

Fancy brownish yellow

I1

E C

C A 0.25

VS2

Clarity

-0.18 ct (-47%/-13%)

Fancy yellow

-0.09 ct (-14%/-11%)

Fancy intense yellow

Fancy intense greenish yellow

I1

0.06 / 79%

Fancy dark brown

Fancy deep greenyellow

SI2

0.15 / 57%

Deemed total loss

0.32

Fancy brown orangy yellow

SI2

-0.14 ct (-28%/-16%)

Fancy yellow

Fancy dark brown

Fancy deep yellowgreen

SI1

0.11 / 65%

VS2

0.28

Fancy brownyellow

I1

-0.10 ct (-25%/-11%)

Fancy intense yellow

Fancy dark greenish gray

Fancy deep yellowgreen

I1

0.11 / 62%

SI1

0.43

Fancy brownyellow

I2

-0.19 ct (-33%/-12%)

Fancy brownish yellow

Fancy dark brownish greenish yellow

Fancy deep greenish yellow

I1

0.12 / 71%

24

ACCEPTED MANUSCRIPT

-------------------Unheated--------------------

After "Lower Dose" Irradiation / LPHT Annealing

--- After LPHT Annealing / Subsequent Repolishing ---

After "Higher Dose" Irradiation / 1000 oC Annealing

Color

Weight (ct)

Color

Weight loss(ct) (% loss Etching / % loss repolishing)

Color

Color

Y-Z, Light brown

0.42

Fancy brownish yellow

-0.09 ct (-12%/-10%)

Fancy intense yellow

Fancy dark yellowish brown

Fancy orangy brown

0.28

Fancy brownyellow

-0.10 ct (-25%/-11%)

Fancy intense yellow

Fancy dark greenish gray

Fancy Dark orangebrown

0.43

Fancy brownyellow

-0.19 ct (-33%/-12%)

Fancy brownish yellow

Fancy dark brownish greenish yellow

Color

T P

I R

C S

U N

After "Higher Dose" Irradiation / LPHT Annealing; Subsequent Repolishing Final weight (ct) / Total % loss

Fancy deep greenyellow

0.24 / 42%

Fancy deep yellowgreen

0.11 / 62%

Fancy deep greenish yellow

0.12 / 71%

A

Graphical abstrct

D E

M

T P

E C

C A

25

ACCEPTED MANUSCRIPT

Highlights   

Detailed study of the effects of low-pressure, high-temperature (LPHT) annealing on type Ia natural diamonds. LPHT annealing alone shows small effect on the diamond color and significant change on surface graphitization and inclusions. Irradiation in combination with LPHT annealing creates a significant shift towards more commercially viable diamond colors.

T P

I R

C S

U N

A

D E

M

T P

E C

C A

26