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.
Synthetic Diamond History
The first batch of
industrial laboratory-grown diamonds were manufactured in 1953 by ASEA, a
Swedish company, but this accomplishment went unannounced and mostly unnoticed.
In 1970, General Electric (GE) produced high-quality synthetic diamond crystals
using the HPHT method with a belt-type of press that created a .78-carat
polished round brilliant colorless synthetic diamond. In the 1980s and 1990s,
Russian scientists used their own technology “BARS” and “TOROID” high-pressure
apparatus to grow industrial and gem-quality crystals up to 2 carats in
polished size and mostly orange to yellow in color. In the past 15 years,
companies including Lucent, Chatham, AOTC and Gemesis — now IIa Technologies —
and many other producers in China, Germany, India, Russia, Ukraine, U.S. and
Taiwan have improved the technology yet again and used their expertise to
successfully grow diamond crystals that result in polished stones from 1 carat
up to 2 carats in size. This “next generation” of diamonds exhibits high
clarities — VS and VVS — and colors D through H, as well as new blue and pink
colors that occur after irradiation. Other companies, including Scio Diamonds,
Washington Diamonds, Taidiam, PDC Diamonds and Pure Grown Diamonds — the
selling arm of IIa Technologies — are also using a very different
technology/process known as Chemical Vapor Deposition (CVD) to produce
laboratory-grown diamonds up to 3 carats in size, of similar quality compared
to HPHT-grown diamonds.
Testing and Identification
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.
UV Fluorescence and Phosphorescence
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.
Strain Patterns
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.
Testing Technique
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.
Practical Tips
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.
Presence of Nitrogen
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.
Summary
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.
About the Authors
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.
Source: diamonds.net
