Identifying Lab-Grown Diamonds
August 8, 2016
Gem-quality synthetic diamonds are more available in today’s jewellery marketplace than ever before, causing both interest and concern among jewellers about the material’s nature and whether it can be identified by gemmologists or gemmological labs.
GIA has studied synthetic or 'man-made' diamonds extensively over the past 30 years, and we know a great deal about how they’re produced and can be recognised. While synthetic diamonds are lab-grown or factory produced, their chemical and physical properties correspond very closely to those of natural diamonds.
Some people might refer to synthetic diamonds as imitations or simulants, but this is incorrect. Imitations like cubic zirconia or synthetic moissanite—which only look like diamond—have very different chemical and physical properties. This allows trained gemmologists to recognise them readily. However, synthetic diamonds are much harder to detect.
In some cases a trained gemmologist can recognise these synthetic and treated diamonds by using standard gem-testing equipment. In other instances, positive identification must involve testing the diamond using advanced scientific instruments at GIA. At GIA we have created a large database of information on the gemmological properties of diamonds of all kinds, which we use to help develop additional means of diamond identification.
HOW DIAMONDS ARE CLASSIFIED BY TYPE
Beginning in the 1930s, scientists began to recognise that certain kinds of diamonds displayed similar features. They grouped the diamonds into two main categories called type I and type II based on differences in transparency under ultraviolet radiation. Scientists were able to further divide type I and type II diamonds into two subcategories by the arrangement of carbon—and impurity—atoms in the diamond structure. In 1959 they discovered that nitrogen was the principal chemical impurity in diamond and that while type I diamonds contained this impurity, type II diamonds did not.
The vast majority of natural diamonds are what scientists call type Ia. Type Ia diamonds contain plentiful nitrogen in clusters or pairs. This kind of diamond cannot be grown artificially. Type Ib diamonds contain scattered and isolated nitrogen atoms that are not in pairs or clusters. Type Ib diamonds are rare in nature. Type IIa diamond contains almost no nitrogen, while IIb diamond contains boron.
Synthetic diamonds correspond to types Ib, IIa, and IIb, all rare categories among natural diamonds.
At GIA type I and type II diamonds can be distinguished by the latter’s transparency under short-wave ultraviolet radiation, and both types can be definitively separated by infrared spectroscopy (The “Type” Classification System of Diamonds and Its Importance in Gemology, Gems & Gemology, Summer 2009, Vol. 45, No. 2).
Natural diamond crystals formed millions—sometimes billions—of years ago deep in the earth, at depths of 100 miles (160 km) or more, and were brought up to the surface much later by explosive volcanic eruptions. These eruptions formed narrow vertical pipes of an igneous rock called kimberlite. Kimberlite pipes are mined to recover the diamonds, and the ore is mechanically broken down to free the crystals. The amount of diamond in kimberlite is very low—perhaps one part per million—so miners must process large amounts of ore to recover the diamonds.
Natural diamonds grow under a range of temperature and pressure conditions. The temperatures are higher than those used to grow synthetic diamonds. At high temperatures, diamonds grow as octahedral crystals, but in the lower temperatures of the laboratory, they grow as crystals with both octahedral and cubic faces. The great age of natural diamonds means that the nitrogen impurities in most diamonds have had time to aggregate into pairs or clusters, making the vast majority—over 95 per cent—type Ia.
Synthetic diamonds are grown over a very short time—several weeks to one month or more—under conditions different from natural diamond formation deep in the earth. Because of the very short growth period, the shape of a synthetic diamond crystal is very different from that of a natural diamond.
Scientists first grew synthetic diamonds in the mid-1950s as tiny crystals. Production of larger crystals suitable for jewellery use began in the mid-1990s and continues today, with more companies becoming involved with diamond growth. Synthetic diamonds are grown in several countries for both jewellery and industrial applications—which may be the more important use for the material (Large Colorless HPHT Synthetic Gem Diamonds from China, Gem News International, Gems & Gemology, Spring 2016, Vol. 52, No. 1).
The traditional synthesis method, called high-pressure, high-temperature (HPHT) growth, involves diamond formation from a molten metal alloy, such as iron (Fe), nickel (Ni) or cobalt (Co). The newer method, referred to as chemical vapour deposition (CVD) or low-pressure, high-temperature (LPHT) growth, involves diamond formation from a gas in a vacuum chamber.
In both methods a diamond crystal or plate is used as a seed to initiate growth.
HPHT diamond growth takes place in a small capsule within an apparatus capable of generating very high pressures. Within the capsule, diamond powder starting material dissolves in the molten metal flux, and then crystallises on the seed to form the synthetic diamond crystal. Crystallisation occurs over a period of several weeks to a month or more to create one or a few crystals.
HPHT synthetic diamond crystals typically show cubic faces in addition to octahedral ones. Because the shapes of natural and HPHT synthetic diamond crystals are different, their internal growth patterns also differ dramatically. These growth patterns can be among the most reliable ways to separate them.
The resulting faceted synthetic gems often exhibit visual features such as colour distribution, fluorescence zoning, and graining patterns related to their cross-shaped, growth-sector structure, as well as the presence of occasional dark flux-metal inclusions. In some cases the material exhibits persistent phosphorescence after the ultraviolet lamp is turned off. These synthetic diamonds can be positively identified using laboratory techniques such as visible and photoluminescence spectroscopy.
Most HPHT-grown crystals are yellow, orangey yellow or brownish yellow. Almost all are type Ib, which is rare in natural diamonds.
Creating colourless HPHT synthetics has been challenging, as modifications to growth conditions and equipment are necessary to exclude nitrogen. In addition, growth rates for high-purity colourless diamond (type IIa or weak type IIb) are lower than for type Ib synthetic diamond, necessitating longer growth times and greater control over the temperature and pressure conditions. While it has traditionally been difficult to grow high-quality colourless HPHT crystals, recent developments have produced crystals sufficient for faceted stones over 10 carats in weight (Large Blue and Colorless HPHT Synthetic Diamonds, Lab Notes, Gems & Gemology, Summer 2016, Vol. 52, No. 2).
The addition of boron in the growth system results in blue crystals. Other colours—such as pink and red—can be produced by post-growth treatment processes that involve radiation and heating, but they are less common.
CVD diamond growth takes place inside a vacuum chamber filled with a carbon-containing gas, such as methane. A source of energy—like a microwave beam—breaks down the gas molecules, and the carbon atoms are attracted downwards to flat diamond seed plates. Crystallisation occurs over a period of several weeks to create a number of crystals at the same time. The exact number depends on the size of the chamber and the number of seed plates. The tabular crystals often have a rough edge of black graphite. They also often display a brown colour that can be removed by heat treatment prior to faceting for gem purposes.
Most CVD crystals are brownish or greyish, but if a tiny amount of nitrogen or boron is introduced into the chamber, yellow, pink-orange or blue crystals can be created. Colourless crystals are easier to produce with this method, but they require a longer time to grow. Most of the CVD-grown colourless material on the market is believed to have been brown crystals decolourised by HPHT annealing. CVD synthetic diamonds are most commonly type IIa.
CVD synthetic diamonds have different gemmological properties to HPHT-grown material. They tend to display even colouration and banded “strain” patterns when observed between crossed polarising filters, and they are of high clarity with few, if any, tiny dark carbon inclusions.
Like HPHT synthesis, CVD synthesis continues to improve and allow manufacturers to offer larger sizes and improved colour and clarity (Recent Advances in CVD Synthetic Diamond Quality, Gems & Gemology, Summer 2012, Vol. 48, No. 2).
GIA recently tested two large CVD synthetic diamonds weighing 2.51 and 3.23 cts, the largest CVD-grown diamonds GIA has examined to date (Two Large CVD-Grown Synthetic Diamonds Tested by GIA, Lab Notes, Gems & Gemology, Winter 2015, Vol. 51, No. 4).
Over the past few years, a growing number of companies have begun production of synthetic diamonds for jewellery use. There have been continued improvements in their clarity and colour, as well as increases in carat weight. Considering the high incidence of synthetic diamonds in news reports, GIA encounters only a small number of synthetic diamonds—apparently submitted unknowingly for GIA grading reports.
To identify gem materials of all kinds, a trained gemmologist uses several kinds of gem-testing equipment, including a refractometer, an ultraviolet fluorescence lamp, a binocular microscope, a polariscope and additional testing tools. As synthetic diamond quality improves further, it’s becoming more challenging to separate them from natural gems using standard equipment.
While even a trained gemmologist may not be able to recognise synthetic diamonds, they can be identified by GIA.
|HPHT Synthetic||CVD Synthetic|
|Uneven colour distribution||Even colour distribution|
|Graining patterns||No graining patterns|
|Unusual fluorescence colours||Unusual fluorescence colours|
|Fluorescence colour patterns||Fluorescence colour patterns|
|Occasional phosphorescence||Occasional phosphorescence|
|Metallic flux inclusions||Occasional dark pinpoint inclusions|
|No strain patterns||Banded strain patterns|
|Possible inscription on girdle||Possible inscription on girdle|
These diagnostic features have been discussed and illustrated in the articles on synthetic diamonds in GIA’s quarterly professional journal, Gems & Gemology. Every issue—past and current—can be viewed and downloaded at the GIA website.
These visual features are characteristic of the majority of synthetic diamonds, but not every faceted synthetic diamond will exhibit all of these features. For example, a particular synthetic diamond may not display any fluorescence. Therefore, it is important to base synthetic diamond identification on as many diagnostic features as possible.
HPHT-grown coloured synthetic diamonds often display uneven colouration that can be seen with transmitted light using the microscope, and if needed, by immersing the cut stone in water or mineral oil to minimise surface reflections. This colour zoning is due to how impurities, such as nitrogen, are incorporated into the synthetic diamond crystal as it forms. Occasionally natural diamonds display some colour zoning, but not in the geometric pattern shown by HPHT synthetic diamonds.
In contrast, CVD-grown synthetic diamonds normally show even colouration.
HPHT synthetic diamonds often display inclusions of solidified flux metal, which appear black and opaque in transmitted light but have a metallic lustre in reflected light. Because the flux metal alloy used for diamond growth usually contains elements such as iron, nickel and cobalt, synthetic diamonds with larger metallic inclusions can be picked up with a magnet.
CVD-grown synthetic diamonds form in a different way, and they do not have metallic inclusions.
Some natural diamonds contain dark inclusions of graphite or some other mineral, but these inclusions do not have a metallic lustre.
When examined between two polarising filters oriented at a 90-degree angle to one another, a natural diamond often exhibits a bright cross-hatched or mosaic pattern of interference or “strain” colours. These interference colours arise from the diamond being subjected to stresses while it was deep in the earth or during its explosive eruption to the earth’s surface. In contrast, synthetic diamonds grow in a nearly uniform pressure environment where they are not subjected to stresses, so when examined in the same way, they display either no strain pattern or a weak banded strain pattern.
The fluorescence of synthetic diamonds is also often quite helpful for identification—it is often stronger under a short-wave than a long-wave ultraviolet lamp, and it can display a distinctive pattern.
HPHT-grown synthetic diamonds tend to show a cross-shaped fluorescence pattern on the crown or pavilion of the cut stone. CVD-grown synthetic diamonds may show a striated pattern when viewed through the pavilion facets. Typical fluorescence colours are green, yellow-green, yellow, orange or red.
When the ultraviolet lamp is turned off, the synthetic diamond may exhibit persistent phosphorescence for up to a minute or more.
At GIA we use a fluorescence imaging instrument called a DiamondView for examining diamonds. This instrument reveals the characteristic growth patterns within diamond crystals.
The real identification challenge facing the jewellery trade is the testing of very tiny diamonds, or melee. This material is sold in parcels of several hundred to several thousand small gemstones, which can include both natural and synthetic diamonds (Screening of Small Yellow Diamond Melee for Treatment and Synthetics, Lab Notes, Gems & Gemology, Winter 2015, Vol. 51, No. 4).
To help the jewellery trade deal with this problem, GIA has recently developed an automated instrument for testing very small diamonds, and we will be introducing a new testing service in our laboratory.
As part of our ongoing research programme on synthetic diamonds, GIA has also recently set up a CVD growth facility where we will be able to produce our own synthetic diamonds for study.
To summarise, synthetic diamonds are now being made available in increasing quantities for jewellery use. Based on more than 60 years of research, we know that synthetic diamonds can be identified, if not by a gemmologist, by the GIA Laboratory. If you have further questions on synthetic diamonds, please contact GIA.
Editor’s note: This article is the summary of a presentation given at the 2016 JCK show in Las Vegas.
Dr Shigley is a distinguished research fellow at GIA in Carlsbad.