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Industry
Record-Size Synthetic Diamond Grown
Unusual bluish orange luminescence is an identifying characteristic of HPHT–grown diamonds from NDT Russia, which has recently grown record-size synthetic diamonds.
A 60-carat high pressure-high temperature (HPHT)–grown diamond set a record for the largest laboratory-grown diamond crystal when it was formed the first week of July 2015 at the New Diamond Technology (NDT) facility in St. Petersburg, Russia. Branko Deljanin, senior gemologist at CGL-GRS, Vancouver, Canada, was visiting at the time and had a chance to examine the synthetic diamond. In April 2015, NDT had experimentally produced a 32.26-carat colorless, high-quality diamond crystal. From this stone, the largest laboratory-grown faceted diamond in the world has been cut. It is a 10.02-carat faceted square emerald-cut synthetic diamond (see Slideshow). NDT has been using the latest Chinese manufactured 850 series cubic presses, which are the biggest cubic presses on the market. As a result, many large stones can be produced in one run. Up to 16 colorless crystals weighing 6 carats to 10 carats can be made in a cycle that takes 10 to 12 days on average. The NDT facility in St. Petersburg has over 50 HPHT presses, including “TOROID”- and “CUBIC”- type presses, which produce 5,000 carats of diamonds per month. NDT built an in-house cutting facility, where the laboratory-grown diamonds are processed and polished.
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Mikko Åström, gemologist and co-founder of M&A Gemological Instruments, Finland, and Deljanin tested six of the largest crystals and polished colorless synthetic diamonds from NDT at the M&A Gemological Instruments facility. Three of the diamonds tested were the record-breaking 10.02-carat E, VS1; a 5.11-carat I, SI1 diamond and a 4.30-carat D, VS2 diamond. Six other samples — including two blue laboratory-grown diamonds — were tested at the GRS laboratory in Hong Kong by Matthias Alessandri, gemologist and advanced instruments operator at GRS Lab (Hong Kong) Limited, China, and Adolf Peretti, founder and chief executive officer (CEO) of GRS Gemresearch Swisslab AG, Switzerland. Both examinations used standard instruments and advanced spectrometers.
As found with other HPHT-grown diamonds on the market, the fluorescence of NDT-grown diamonds with short-wave ultraviolet (SWUV 254 nm) were greenish blue and more intense than with long-wave ultraviolet (LWUV 365 nm). This characteristic was observed in half the samples. Almost 90 percent of natural diamonds usually show some blue reaction under UV light and, in contrast to synthetics, fluoresce more intensely to LWUV than to SWUV. No natural diamonds, but most synthetic diamonds have more intense fluorescence under SWUV than LWUV light. An interesting new feature discovered in approximately half the samples, which were from NDT’s latest production in 2015, is a weak-medium bluish orange fluorescence and phosphorescence under an SWUV lamp (see Slideshow). This phenomenon was also present when the samples were exposed to a strong LED white light. All colorless laboratory-grown diamonds tested were type IIa with no presence of nitrogen and a small amount of boron. The blue laboratory-grown diamonds (see Slideshow) were type IIb, containing boron — responsible for the blue color — in higher concentrations than found in natural type IIb blue diamonds. Grayish blue CVD-grown diamonds could be either type IIb or IIa. Most NDT-grown diamonds studied exhibit high to medium clarity — VVS1to SI1. Based on this clarity, it is not possible to distinguish the majority of samples from similar-quality natural diamonds by using just a loupe or microscope. Only a few lower-clarity stones — especially unpolished — had metallic inclusions that are typical for HPHT-grown diamonds.
Diamond is an isotropic material, which means that it exhibits the same properties in all directions, but it has anomalous double refraction (ADR) resulting from “strain” which is visible as a rainbow effect pattern under cross-polarized filters (CPF) (see Slideshow). One cause of strain in natural diamonds is the internal deformation of the crystal structure caused by heat and pressure from geological processes. As a result of the strain, a single light source is reflected in two directions within the diamond. Diamond cutters are aware of strain in natural diamonds and are very careful when cutting diamonds exhibiting high strain because they could break under heat/pressure produced while cutting. Strain patterns are visible under CPF.
Using the filter technique, a diamond may be tested by positioning it between CPF or in a portable polariscope and rotating the diamond while observing patterns in transmitted light with a loupe or microscope. Diamonds of different type and origin have different strain patterns under CPF:- Low-nitrogen natural diamonds — type IIa — often have a typical “tatami pattern,” which are fine intersecting parallel dislocations.
- Natural diamonds that contain nitrogen — type Ia, comprising 95 percent of natural diamonds — have a strong strain pattern showing a multicolor rainbow effect.
- Synthetic diamonds grown by the HPHT method are not subjected to strong geological forces and so these synthetic stones are free of strain. An absence of any strain pattern under CPF is confirmation of synthetic origin. In the case of NDT, these are type IIa colorless or type IIb blue diamond grown using HPHT methods.
When examining a diamond for origin, it is important to use standard instruments such as a microscope with CPF or a portable polariscope and a UV lamp. The presence or absence of strain patterns is an indication of the diamond origin. They are present in natural diamonds, but absent in HPHT-grown diamonds. It is also necessary to check for the presence of blue fluorescence, a property of 90 percent of natural diamonds, or the absence of blue fluorescence, which is a characteristic of all synthetic diamonds and 10 percent of natural diamonds when viewed under an LWUV lamp. Additional testing at a gem lab will confirm the nature of the diamond.
Defect centers comprising nitrogen-vacancies (NV) like NV0 (575 nm) and NV- (637 nm) and silicon-vacancies Si-V- (737 nm) are present in approximately half the NDT samples. Si-V defects are found in CVD-grown diamonds and are very, very rare in natural diamonds. Nickel was not detected in the samples and if present, probably has a low concentration below the sensitivity of the photoluminescence (PL) spectrometers used at GRS and at M&A Gemological Instruments. In the opinion of the authors, orange color luminescence is most probably from NV centers, maybe in combination with other defects. This is most likely a characteristic of very large stones resulting from the long-lasting growth period when stones were exposed to high pressures and high temperatures.
Diamonds grown by NDT are the largest colorless synthetic diamonds reported to date, with weights up to 60-carat crystals and 10-carat polished. Most samples are high color — D to I range — and high to medium clarity, but may contain metallic inclusions formed from metal/catalyst melt. Natural and CVD synthetic diamonds show strong Ia or weak IIa strain pattern as they are more heavily strained than HPHT synthetic diamonds from NDT, which do not show strain pattern under CPF. Colorless samples were type IIa with some boron — weak type IIb — or blue with high boron content, so it is possible to distinguish them from natural stones of similar color. All samples fluoresced and phosphoresced blue and half of them orange when exposed to a UV lamp, with stronger responses to short-wave than long-wave excitation and lasting phosphorescence. Natural diamonds show a stronger reaction — usually blue — under LWUV than SWUV light. Only rare chameleon and type IIb natural diamonds phosphoresce, but not with a blue/orange color, but yellow for chameleon and red for blue diamonds. Photoluminescence spectra revealed NV centers, which in combination with other defects could be responsible for orange luminescence. Boron impurities are responsible for the blue phosphorescence in HPHT-grown diamonds but the exact nature of orange phosphorescence is still not fully understood, and further research is underway at GRS and CGL-GRS labs to also understand the role of boron and other impurities present. Using a combination of standard gemological and spectroscopic tests, it is possible to identify all NDT colorless and blue HPHT-grown diamonds and thereby distinguish them from natural diamonds of similar quality. The authors predict that in the future large-sized HPHT-grown pink diamonds will be produced using Beta irradiation in combination with high temperature treatment from type Ib yellow starting material, similar to the process reported (“Contributions to Gemology” No. 14, pages 21 to 40) to obtain the pink color in CVD-grown diamonds. Deljanin has had the opportunity to study HPHT-grown and CVD-grown samples from all producers of synthetic diamonds that openly sell to the jewelry industry in the past 15 years and this is the first time that HPHT-grown diamonds are exhibiting an orange luminescence, which is usually a characteristic of CVD-grown colorless diamonds under SWUV light. The most surprising phenomena is that even visible pinpoint light triggers this orange reaction, indicating that it could be a straightforward and simple “screening test” for most larger synthetic diamonds coming from NDT, Russia.
Branko Deljanin is president and senior gemologist at CGL-GRS, Vancouver (Canada), info@cglworld.ca. Matthias Alessandri is gemologist and operator of advanced instruments at GRS Lab (Hong Kong) Limited, Hong Kong (China). Dr. Adolf Peretti is founder and CEO of GRS laboratories in Switzerland, Hong Kong, Thailand and Sri Lanka and partner of CGL-GRS in Canada. Mikko Åström is gemologist and co-founder of M&A Gemological Instruments (MAGI), Järvenpää (Finland). Dr. Andrey Katrusha is consultant and scientific adviser to NDT, St. Petersburg, Russia.Article from the Rapaport Magazine - November 2015. To subscribe click here.
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