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In-Depth
Diamond Detectives
Gemologists: “If it’s lab-grown, we can detect it.”
By Shuan Sim
As lab-grown diamonds grow bigger and better with technological improvements, the need for better detection of such diamonds grows as well. Some people in the diamond trade worry about unscrupulous traders trying to pass off lab-grown diamonds as natural given that diamonds created in laboratories these days have grown whiter and clearer. Gemologists, however, are confident that they are able to detect any lab-grown diamond that passes their way. That is not to say that gemologists believe lab-grown diamond producers are playing a cat-and-mouse game with laboratories to escape detection. “I believe they’re trying to improve the stones to make them as pretty as they can, not trying to confuse me,” comments David Weinstein, head gemologist at the International Gemological Institute (IGI).
Major gem-testing labs around the world adopt similar procedures to test diamonds to ascertain whether or not they are natural or lab-grown, simulants or treated, although each lab might use different methods and machines to achieve the goal.- All colorless diamonds first undergo screening and are currently tested to see if they are type Ia — which comprises about
- 98 percent of all natural white diamonds. Any stone that is determined to be type II will require more testing. Different methods of screening include infrared (IR) spectroscopy, ultraviolet (UV) absorption, UV transparency and photoluminescence (PL) imaging. These machines can identify natural diamonds with about 97 percent accuracy and about 3 percent of natural diamonds will be flagged for further testing. Type Ib diamonds — deep yellow and brown visually — tend to go straight to finalizing processes.
- Diamonds flagged for further testing will undergo a finalizing process, where the stones are examined by a person under machines that examine element composition, crystal structures, electromagnetic absorption, phosphorescence and fluorescence, to name a few.
- IR spectroscopy measures how different elements in a diamond absorb electromagnetic radiation at different wavelengths. Each type absorbs infrared light at specific wavelengths.
- The ultraviolet absorption method, similar to infrared spectroscopy, measures whether the diamonds absorb UV rays at the specific wavelength that type I diamonds do.
- The UV transparency method measures whether specific bands of light can pass through a diamond. Natural type Ia diamonds are opaque to this light, while type IIa — which appears colorless — are transparent.
- PL imaging comprises testing for fluorescence — how a diamond glows in the presence of UV light — and the strength and duration of phosphorescence — a diamond’s ability to continue glowing after the UV source has been shut off. Natural type Ia diamonds tend to fluoresce blue and do not phosphoresce, and any diamond that glows in other colors or continues glowing are referred for further testing.
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Several types of equipment that are used by gem-grading laboratories also exist on the market commercially for screening and analysis of lab-grown diamonds. This type of equipment can range from simple handheld devices to specialized machinery for screening in bulk and providing further analysis. Screening devices, which identify a stone as a natural diamond or state that it requires further testing, include:- DiamondSure, from De Beers, uses UV absorption to rapidly screen colorless or near-colorless diamonds weighing .10 carats up to 10 carats, mounted and loose stones, to determine whether they are natural, treated or simulants. Measurements take a few seconds per stone.
- DiamondPlus, from De Beers, is primarily designed to detect HPHT treatments, though it is also capable of detecting CVD diamonds, through laser-excited PL analysis. Samples, ranging from .05 carats up to 10 carats, have to be immersed in liquid nitrogen.
- Automated Melee Screening (AMS), from De Beers, uses UV absorption to screen melee stones weighing .01 carats to .20 carats for lab-grown diamonds and simulants. The machine holds up to 500 carats and its throughput is approximately 360 stones per hour.
- DiamondCheck, from GIA, uses IR spectroscopy for rapid screening of diamonds weighing .01 carats to 10 carats. Unlike other machines that are designed for white and near-colorless diamonds, DiamondCheck can also screen pink diamonds, provided any possible coatings are removed first. DiamondCheck also identifies HPHT-treated stones and simulants.
- D-Screen, from HRD Antwerp, uses UV transparency to screen whether or not diamonds are natural, HPHT-treated or lab-grown. D-Screen is not used to distinguish simulants and is for diamond colors D through J.
- M-Screen, from HRD Antwerp, uses UV transparency to screen melee stones in colors D through J weighing .01 carats to .20 carats for lab-grown diamonds, simulants and treatments. HRD Antwerp says the machine can process melee at a rate of anywhere from 7,200 stones to 10,800 stones per hour.
- GV5000, from the National Gemstone Testing Center (NGTC) in China, uses UV PL analysis to examine loose and mounted melee from .002 carats to 8 carats. Besides being able to process smaller stones than other machines, the GV5000 can also work with stones that have strong fluorescence, are faint yellow or are mounted in jewelry. Its throughput of .01-carat stones is approximately 12, 000 per hour.
Diamonds that do not pass the screening test will have to be finalized by a gemologist. The equipment used for final testing that are commercially available include:- DiamondView, from De Beers, examines PL images that are produced by exposure to shortwave UV illumination. The machine tests diamonds .05 carats up to 10 carats, as well as melee weighing .01 carats up to .20 carats. It is designed for loose stones, but adapters for certain mounted jewelry are available.
- Alpha Diamond Analyzer, from HRD Antwerp, uses IR spectroscopy to analyze diamond types — such as types I and II. It also tests whether or not a diamond is lab grown, treated or a simulant.
Various gem laboratories around the world not only offer diamond detection services but also offer grading reports for lab-grown diamonds. GIA’s lab-grown diamond reports differ from their regular reports in that they use broad categories such as “colorless,” “near colorless” for color and “VS” and “SI” grades without numerical distinctions. According to the GIA’s site, the lack of specificity in its grading is because lab-grown diamond production has yet to show the ability to grow diamonds at that level of distinction. Tom Moses, executive vice president and chief laboratory and research officer at GIA, adds that diamond growers aren’t interested in synthesizing lower colors. “We might change it in the future if we see a greater need for granularity and producers were able to grow at specific levels, but right now there is no need.” Other labs grade lab-grown diamonds with the same level of specificity natural diamonds receive.
“Synthetic diamonds are indistinguishable to the naked eye,” says Moses. “It might be possible to spot a synthetic diamond under a loupe, if it contains specific visible inclusions, especially with HPHT diamonds,” he notes, explaining that the manufacturing process of HPHT diamonds might leave metallic inclusions. However, with many polished HPHT diamonds, the metallic inclusions would have been accounted for during the cutting process, effectively eliminating or obscuring the visibility of such flaws. The examiner would also have to know what he or she is looking at or be looking out for them, Moses continues. Loupes would not help with CVD diamonds as they lack similar inclusions.
There are some basic tools that can be used in the detection of lab-grown stones. These could range from simple magnets to handheld “diamond testers” that test for thermal and electrical conductivity, densities and refraction. Another method includes the use of a regular UV lamp, cross-polarizing filters and a microscope — a simple setup that yields accurate results but is often overlooked by jewelers, says Deljanin. These methods can provide quick identification for lab-grown diamonds and simulants with telltale signs. However, points out Moses, “Most detection of lab-grown diamonds requires specialized equipment, and they provide the most assurance.”
To accurately determine whether a flagged diamond was a false positive, or if it were indeed lab-grown and what kind of synthetic diamond it is, multiple machines are needed to look at different factors. A machine is usually designed to focus on one aspect of a diamond’s characteristics. “There is no one magic machine that can do everything at once,” notes Deljanin.
HPHT diamonds and CVD diamonds have fundamentally different characteristics:- Crystal structures: HPHT diamonds have cuboctahedral growth, displaying an “hourglass” pattern when illuminated, while CVD diamonds have layered growth, displaying striations when illuminated. (see The Technology of Lab-Grown Diamonds, page 44). Natural diamonds possess triangular ring-like growth patterns, also called “trigons.” These growth patterns become visible when the diamond is exposed to high-intensity UV rays, causing the diamond to fluoresce, or emit light after having absorbed the radiation. “Hourglass” patterns are telltale signs of HPHT diamonds, though in 2013, GIA encountered an exceedingly rare case of a natural diamond also exhibiting cuboctahedral growth.
- Besides growth patterns, strain patterns can be examined when viewed through cross-polarizing filters. These strain patterns cause a double refraction of light. HPHT diamonds typically display little to no levels of strain. Natural diamonds usually display “tatami” or mottled strain patterns due to their formation processes. However, CVD diamonds can also display strain patterns similar to natural diamonds.
- Photoluminescence: Natural white and near-colorless diamonds can fluoresce blue with no phosphorescence.They can also fluoresce green to yellow depending on the color of the diamond. White or near-colorless HPHT diamonds tend to display greenish-blue fluorescence, with strong greenish-blue phosphorescence for three seconds and even up to a minute after the UV source has been switched off. According to Lan Yan, director of NGTC, China, his research center has not found any HPHT diamonds capable of fluorescing blue and also with no phosphorescence. CVD diamonds can fluoresce greenish-blue, green, red, orange or not at all and can display anywhere from weak to strong phosphorescence lasting less than three seconds.
- Inclusions: Under a microscope, HPHT diamonds can possess visible metallic inclusions that are often rod-shaped, though they can also take on plate and irregular shapes. These inclusions are typically a by-product of the metal solvents used in the growth process. CVD diamonds, because of the greater control over growth environments, are usually inclusion-free, though they could have black nondiamond carbon inclusions, usually confined to the growth planes. GIA has reported CVD diamonds with inclusions that sometimes resemble natural diamond inclusions.
Although diamond-growing technology has come a long way since its debut back in the 1950s, the biggest advances really took off in recent years. Being able to grow type IIa/b HPHT diamonds only hit the market as recently as 2014, according to GIA. In time, Deljanin strongly believes the industry will only see bigger and clearer lab-grown diamonds. “Diamond-growing technology has really only developed recently with the jewelry sector in mind,” he concludes.Article from the Rapaport Magazine - April 2016. To subscribe click here.
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