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Identification of a CVD synthetic diamond with a 'tree ring' growth pattern.


During the last few decades, a considerable number of gem-quality synthetic diamonds produced by chemical vapour deposition (CVD) have entered the market. Most of them have reportedly come from Apollo Diamond Inc., Gemesis Corp. and others (Wang et al., 2003, 2007b, 2012; Martineau et al., 2004; Wang and Moses, 2011). In 2003, the Gemological Institute of America (GIA) laboratory described a near-colourless gem-quality CVD synthetic diamond grown by Apollo Diamond (Wang et al., 2003). Subsequently, Wang et al. (2007b, 2010) reported on colourless, brown and orange-pink gem-quality CVD-grown synthetics from Apollo Diamond. Martineau et al. (2004) presented analytical results from the Diamond Trading Co. (DTC) Research Centre on CVD synthetic diamond samples grown for research purposes by Element Six; these included as-grown and HPHT-treated nitrogen-doped samples, as well as boron-doped and high-purity CVD synthetic diamonds. Gemesis Corp. announced plans to market CVD-grown synthetics in November 2010, and these were described by GIA in 2011 (Wang and Moses, 2011). Subsequently, Wang et al. (2012) reported additional gemmological and spectroscopic properties of Gemesis CVD synthetic diamonds.

In recent years, greater identification challenges for CVD synthetic diamonds have resulted from refinements in their growth and processing, which have yielded products that are very similar to natural diamond in appearance, with variable properties according to the added impurities or post-growth treatment. In the past few years, undisclosed CVD synthetic diamonds have been submitted to the International Gemological Institute, National Gemstone Testing Centre (NGTC), Tokyo Central Gem Laboratory and others (e.g. Song et al., 2012; Kitawaki et al., 2013, 2015).

NGTC has carried out significant research on CVD synthetic diamonds, and has developed effective identification protocols using ultraviolet-visible-near infrared (UV-Vis-NIR) and infrared absorption spectroscopy, the DiamondView instrument, PL spectroscopy, etc. Of these, DiamondView fluorescence imaging is one of the most useful identification methods, since it reveals the striations associated with the layered growth of CVD synthetics (Martineau et al., 2004; Wang et al., 2010). However, this article documents a 'tree ring' growth pattern seen with the DiamondView in the table of a faceted CVD synthetic diamond that is similar to patterns seen in some natural diamonds. The gemmological characteristics of this sample are presented, and the reason for the unusual appearance of its growth pattern is analysed using X-ray topography and Laue diffraction to determine the crystallographic orientation of the synthetic diamond relative to the table facet.

Materials and Methods

A 0.61 ct round brilliant (Figure 1) was submitted to NGTC's Shenzhen Laboratory for a grading report in February 2015. It was identified as a CVD synthetic diamond, and received a low colour grade (L) but showed good clarity ([VVS.sub.2]). The following investigations were carried out at NGTC, except for X-ray topography and Laue diffraction, which were carried out at De Beers Technologies in Maidenhead.

The sample's fluorescence and phosphorescence to UV radiation were observed using 254 nm (short-wave) and 365 nm (long-wave) 4-watt UV lamps, and also with the DiamondView deep-UV (<230 nm) imaging system.

Infrared absorption spectra were recorded in the mid-infrared range (6000-400 [cm.sup.-1] and 2 [cm.sup.-1] resolution) at room temperature, with a Nicolet 6700 Fourier-transform infrared (FTIR) spectrometer equipped with a KBr beam splitter. UV-Vis-NIR absorption spectra in the range of 230-1000 nm were collected with an Ocean Optics GEM-3000 Jewel Identifying Instrument at liquid-nitrogen temperature. The sample was also tested using the DTC DiamondSure and DiamondPLus instruments.

Photoluminescence spectra were acquired with four different laser excitations (325, 473, 532 and 785 nm) using a Renishaw InVia Raman confocal micro-spectrometer with the sample cooled to liquid-nitrogen temperature.

X-ray topography (Mo K[alpha] radiation, {533} reflection) using a Marconi GX20 rotating anode X-ray generator was done to visualize strain associated with dislocations that formed during the sample's growth. The crystallographic orientation of the table facet was determined by Laue diffraction, using a Bruker SMART 1000 CCD single-crystal diffractometer. Laue diffraction patterns are useful for revealing the crystallographic orientation of a faceted sample. Named after Max von Laue, such images provide a photographic record of the diffraction pattern that is produced when an X-ray beam passes through a crystal (Warren, 1969). The patterns show a regular array of spots on a photographic emulsion resulting from X-rays scattered by certain groups of parallel atomic planes within the crystal. Those X-rays oriented at just the proper angle to a group of atomic planes will combine in-phase to produce intense, regularly spaced spots on the film or plate that indicate the sample's crystallographic orientation relative to the X-ray beam.



The sample was inert to long-wave UV radiation and showed moderate-to-strong green fluorescence to the short-wave UV lamp. No obvious phosphorescence was seen.

In the DiamondView, the sample displayed bluish green fluorescence with layered growth striations (Figure 2). Unlike previously reported CVD synthetic diamonds, when observed table-up the sample showed a 'tree ring' growth pattern, similar to that seen in some type I natural diamonds (Wang et al., 2007a; Sun et al., 2012; see Figure 2a). By contrast, when viewed from the pavilion the sample showed the characteristic parallel layers associated with the growth of CVD synthetic diamond (Figure 2b). As far as we are aware, the 'tree ring' pattern has not been previously reported in CVD synthetic diamond (cf. Martineau et al., 2004; Wang et al., 2007b).

Infrared and UV-Vis-NIR Absorption Spectroscopy

The absorption spectrum in the mid-infrared region (Figure 3) showed that the sample was type IIa, with very weak N-related absorption at 1344 [cm.sup.-1]. Other features included H-related lines at 3107, 3029, 2919, 2880 and 2831 [cm.sup.-1], and weak bands at 1340, 1332, 1296 and 1128 [cm.sup.-1]. Note that the weak line at 3107 [cm.sup.-1] and the missing 3123 [cm.sup.-1] line (a characteristic H-related feature in CVD synthetic diamond) provide strong evidence that the sample had been HPHT treated (Martineau et al., 2004).

The UV-Vis-NIR absorption spectrum revealed an N-related peak at 270 nm and decreasing absorbance from 300 to 700 nm (Figure 4). Neither the Si-[V.sup.-]-related peak at 737 nm (typical of some CVD synthetic diamonds: Martineau et al., 2004; Wang et al., 2012) nor the N3 absorption at 415 nm (common in natural diamond) were detected.

Verification Instruments

Testing with the DiamondSure resulted in 'refer for further test: type II', and the DiamondPLus indicated 'Refer CVD' from the PL measurement of the 737 nm Si-[V.sup.-] centre.

Photoluminescence Spectroscopy

325 nm Excitation: A weak PL peak at 415.1 nm was excited by the 325 nm laser (Figure 5). This peak is attributed to the N3 defect (three nitrogen atoms in a {111} plane associated with a vacancy: Davies, 1974; Davies et al., 1978), which usually occurs in cape diamonds. Nitrogen in CVD synthetic diamond is commonly present in the form of single nitrogen, but may be aggregated during HPHT treatment to produce the N3 defect, particularly after prolonged annealing at 2,200[degrees]C (Martineau et al., 2004). In addition, a series of weak PL peaks in the 388-503 nm range were detected.

473 nm Excitation: The PL spectrum excited by the 473 nm (blue-green) laser is shown in Figure 6. The strongest zero-phonon-line (ZPL) emissions were at 575, 503.2 and 736.6/736.9 nm.

The 575 nm emission is from the N[V.sup.0] centre, which is commonly detected in CVD synthetic diamond due to the presence of single nitrogen and vacancies that are inevitably introduced during the growth process (Martineau et al., 2004).

The 503.2 nm ZPL is attributed to the H3 defect ([[N-V-N].sup.0]), which can be produced by irradiation in diamond containing aggregated nitrogen, followed by annealing at approximately 800[degrees]C. Vacancies are generated during the irradiation and annealing, and are trapped at nitrogen A-aggregates (a nearest-neighbour pair of nitrogen atoms) to form H3 centres. In CVD synthetic diamond, this defect is produced when there is nitrogen and a source of vacancies in the pre-treated material. H3 defects also can be formed by high-temperature annealing without irradiation (Charles et al., 2004; Martineau et al., 2004; Meng et al., 2008). According to Meng et al. (2008), when the annealing temperature exceeds 1,700[degrees]C, H3 defects may be observed in CVD synthetic diamond. This defect is responsible for the green fluorescence excited by the short-wave UV lamp and the DiamondView (Wang et al., 2012).

The 736.6/736.9 nm doublet and its corresponding vibronic structure are attributed to the Si-[V.sup.-] centre. Silicon is often introduced into CVD synthetic diamond by the etching of Si-containing components forming the reactor (Robins et al., 1989; Barjon et al., 2005; Wang et al., 2012). The defect can exist stably after HPHT treatment (Martineau et al., 2004), and is usually regarded as one of the identification characteristics of CVD synthetic diamond (although not definitive, as a small number of natural diamonds also have this defect: Breeding and Wang, 2008).

The 473 nm laser also excited many unidentified sharp peaks, including those at 487, 488, 494, 495, 500, 501, 506 and 507 nm, as well as those at 975, 977, 983, 988 and 990 nm in the infrared region (not all peaks are shown in Figure 6).

532 nm Excitation: Photoluminescence peaks at 575 nm ([NV.sup.0]) and 736.6/736.9 nm (Si-[V.sup.-]) were detected in the sample when excited by the 532 nm laser (Figure 7). In addition, 637 nm emission attributed to the [NV.sup.-] centre was recorded. Several unattributed emission peaks were detected in the 539-556 nm region, including those at 539.3, 540.1, 540.6, 541.6, 544.8, 547.7, 548.7, 552.5, 554.2 and 556.0 nm (not all peaks are shown in Figure 7).

785 nm Excitation: Weak PL peaks at 819.3, 824.5, 850.1 and 853.3 nm were detected in the sample when excited with the 785 nm laser (Figure 8). The defects responsible for these PL peaks have not been identified.

X-ray Topography

The X-ray section topograph of the sample is similar to those previously reported in CVD synthetic diamond (Martineau et al., 2004), showing a columnar texture with linear contrast streaks parallel to the growth direction (Figure 9). This appearance is quite different from that seen in topographs of natural diamonds (cf. Diehl and Herres, 2004). The contrast streaks are believed to be caused by the dislocations produced during the growth process. If the X-ray beam sampled a direction perpendicular to the growth direction, these dislocation bundles would appear as dark spots in the resultant topograph (Martineau et al., 2004). The X-ray topograph shows that the table of the faceted CVD synthetic diamond is oriented at a significant oblique angle to the crystal's growth direction, in contrast to that reported by Martineau et al. (2004) where the table was approximately perpendicular to the growth direction.

Laue Diffraction

A Laue diffraction image taken with the table parallel to the plane of the image (Figure 10) showed that the table facet of the CVD synthetic diamond was oriented approximately 20[degrees] to the {111} octahedral plane, in the {100} cubic direction.


The angle between the octahedral and cubic planes in diamond is 54.7[degrees]. Laue diffraction allows the orientation of the CVD synthetic diamond's table facet to be calculated (Figure 11), showing a deviation from the {100} cubic plane toward the {111} octahedral plane. In general, CVD synthetic diamonds are grown on plates oriented parallel to the {001} plane, producing growth in the <001> direction. The table facet is typically oriented approximately parallel to the seed plate plane (i.e. perpendicular to the growth direction of CVD synthetic diamond) to improve the cutting yield. The Laue diffraction pattern indicates that the 'tree ring' growth pattern observed on the table of the CVD synthetic diamond sample with the DiamondView is due to the orientation of the table at just the right angle to the growth direction to produce very shallow angles between the growth planes and the table and crown facets at one position at the edge of the table. This results in the concentric ring-like appearance where the striations intersect the facets.


Refinements in CVD synthetic diamond growth technology, and the resulting increase in crystal size, have resulted in more flexibility regarding the cutting orientation used to facet such products. The 'tree ring' growth structure observed with the DiamondView in the table of a 0.61 ct CVD synthetic diamond is due to the crystallographic orientation of the table facet deviating from the {100} cubic direction toward the {111} octahedral plane to produce very shallow angles between the growth planes and the table and crown facets. This concentric pattern is different from the dislocation and 'mosaic' structures seen in DiamondView images of natural type IIa diamond (Martineau et al., 2004), but may resemble those patterns observed in natural type Ia stones; however, the luminescence of the latter gems shows a blue colour that is very different from that seen in CVD synthetics (Wang et al., 2007a; Sun et al., 2012). Also, compared to the regular striations in CVD synthetics, the growth patterns in natural diamond are more complicated and less orderly because of the complex geological environment of their formation. In addition, the CVD sample showed the Si-[V.sup.-]-related doublet at 737.6/737.9 nm in PL spectra. This defect can be used to identify CVD synthetic diamond but is also seen very occasionally in natural diamond. The columnar texture of the X-ray topograph may be useful as an indicator of CVD synthesis (cf. Diehl and Herres, 2004; Martineau et al., 2004). The presence of the N3 (415 nm) and H3 (503.2 nm) defects, and also the 3107 [cm.sup.-1] absorption seen in the mid-IR spectrum, indicate that the CVD synthetic diamond has been HPHT treated. The 480-524 nm PL peaks excited by the 473 nm laser and the 535-560 nm PL peaks excited by the 532 nm laser have also been detected in other CVD synthetic diamonds that show green fluorescence and characteristics of HPHT treatment, and may be used as further indicators of CVD synthetic diamond.


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The Authors

Yan Lan, Rong Liang, Tianyang Zhang and Hong Ma

National Gemstone Testing Center (NGTC)

Gems & Jewelry Institute of Shenzhen

13F NGTC Building, 4 # Beili South Road

Luohu District, Shenzhen 518020, China


Yong Zhu and Xuan Wang

Chongqing Academy of Metrology and Quality

Inspection, 1 # Yangliu North Road

Yubei District, Chongqing 401123, China

Dr Taijin Lu, Jian Zhang and Zhonghua Song

NGTC, Gems & Jewelry Institute of Beijing

22F Tower C, Global Trade Center

36 # North Third Ring East Road

Dongcheng District, Beijing 100013, China


Special thanks to John Freeth and David Fisher of De Beers Technologies, Maidenhead, who provided assistance with the X-ray diffraction and topographic analysis of the sample.
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Article Details
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Title Annotation:Feature Article
Author:Lan, Yan; Liang, Rong; Lu, Taijin; Zhu, Yong; Zhang, Tianyang; Wang, Xuan; Zhang, Jian; Ma, Hong; So
Publication:The Journal of Gemmology
Article Type:Report
Date:Dec 1, 2015
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