Editor’s Note: The earliest laboratory methods for creating synthetic diamond were cost prohibitive. Technological advances, however, have made it easier for synthetic diamond to enter the marketplace. Unfortunately for consumers, proper diamond testing is quite expensive, making it difficult to know whether many diamonds are natural or artificial. This five-part series of articles, “Understanding Gem Synthetics, Treatments, and Imitations,” is a chapter from Dr. Joel Arem’s forthcoming book, Gems and Jewelry, 3rd Edition. © Joel E. Arem 2011-2013. The International Gem Society (IGS) gratefully thanks Dr. Arem for his contributions to the field of gemology and for allowing us to reproduce this chapter.
Table of contents:
- Special Methods
- Synthetic Diamond
- Types Of Diamond
- HPHT Process
- Understanding Gem Synthetics, Treatments, And Imitations, Part 1: An Introduction
- Understanding Gem Synthetics, Treatments, And Imitations, Part 2: Crystal Growth
- Understanding Gem Synthetics, Treatments, And Imitations, Part 3: Synthetic Diamond
- Understanding Gem Synthetics, Treatments, And Imitations, Part 4: Synthetic Gemstone Guide
- Understanding Gem Synthetics, Treatments, And Imitations, Part 5: Identifying Gemstone Treatments
The previous discussion [Part 2] encompasses the overwhelming majority of materials. However, some crystals require very unusual growth conditions. (This was certainly true of cubic zirconia until skull melting became a commercial process!). Perhaps the most notable of these is diamond.
Diamond is a product of extremely high temperatures and pressures, conditions that are chiefly found in the Earth’s mantle at a depth of 15 or more miles below the surface. The major obstacle to diamond synthesis was finding (1) equipment that could produce these conditions and (2) materials to use in making this equipment that would allow the equipment itself to survive (and maintain) these conditions! Success was claimed often in the period 1850-1950, but never truly documented. That situation has changed, dramatically and irreversibly, over the past 60 years.
Diamond is made of pure carbon. The hardness and optical properties of diamond are unique because of its compact, tightly bonded crystalline structure. The conditions needed to produce this structure were for many years believed to be very high temperature and pressure, similar to the growth environment for natural diamonds. Attempts to duplicate nature’s process in the laboratory as early as the 1880s (J.B. Hannay, in Glasgow, and in 1896 by Henri Moissan in France) were inconclusive.
Unquestionable proof of diamond synthesis was, in fact, not forthcoming until 1955, when scientists at General Electric Co. announced a breakthrough. A 1,000-ton press achieved a simultaneous temperature of 5,000°F and pressure of 1.5 million pounds per square inch. Diamond forms from carbon very quickly (seconds to minutes) at these conditions. The largest crystals grown by GE were about one carat and a few small gems were cut from them. The manufacturing cost, however, was so high, and the process so difficult, that synthetic diamond gems could not, at that time, compete with natural stones. By contrast, synthetic diamond powder for abrasives was relatively easy to produce and has now become a staple and quite inexpensive industrial product, but the production of large, transparent diamond crystals suitable for gemstone use remained technologically elusive and very expensive.
As long as the conditions of extreme temperature and pressure that exist at great depth, where diamond forms within the earth, were not attainable in a laboratory or industrial environment, the diamond trade remained immune to the devastating effects of undisclosed synthetics and treatments, problems that have plagued the rest of the gemstone marketplace.
Curiously, the impetus to make large diamonds did not come primarily from gemstone demand. It turns out that diamonds are among the best-known conductors of heat. The limitation on miniaturization of microelectronics is the dissipation of heat generated by electrons moving in these ultra-small circuits. The CPUs in modern computers generate so much heat that they even have their own dedicated cooling fans! It had long been speculated (and was later verified) that a huge increase in component density could be achieved by using diamond as a substrate instead of silicon, leading to a revolutionary advance in miniaturization. The enormous size of this potential electronics market, totally dwarfing any possible gemstone use, inspired significant advances in diamond crystal growth methods.
One of these, known as CVD (chemical vapor deposition) was initially projected to be capable of growing transparent single diamond crystals as large as 2-inch cubes! The long-term potential therefore now exists for making polished gem diamonds of almost any desired size. This (relatively unrecognized) situation has profound implications for the jewelry industry.
Types Of Diamond
Diamonds are grouped into several categories. Type I stones contain nitrogen, either in clusters (type IA) or as isolated atoms (type IB). All type I stones are electrical insulators and have absorption bands within the infrared portion of the spectrum. They commonly fluoresce yellow in ultraviolet light.
Type I stones can be colorless, grayish or green (rarely blue or violet), but are normally yellowish to brownish, and include the Cape Series of yellowish stones that comprise the vast majority of diamonds seen in the gemstone trade. Type II diamonds do not contain nitrogen and are good conductors of heat. Type IIA stones are colorless, but IIB ones contain boron, have a bluish (sometimes brownish or gray) color, and are electrically conductive. A very rare category called HGBV (hydrogen-rich, gray to blue to violet color) is produced by the Argyle Mine in Australia. HGBV stones contain hydrogen and also nickel, which is extremely rare in diamonds from other localities. Bluish type I stones are extremely rare, and can be distinguished from boron-rich type IIB stones because they are non-conductive and have different fluorescence.
Some diamonds are “allochromatic” and colored by impurities, such as nitrogen and boron, which create “color centers” that selectively absorb certain light. However, an enormous range of hues is created by various kinds of structural defects, such as vacancies. These occur naturally but can also be produced by a wide range of energetic processes.
In 1904, notable physicist William Crookes buried some diamonds in a packing of radium salts. In a relatively short time the stones turned green. Unfortunately, they also became intensely radioactive! Fast forward to the 1940s – by now, particle accelerators were invented and widely used in high-energy physics experiments. It was discovered that cyclotron bombardment (with protons, alpha particles and deuterons) could color diamonds green, blue-green, and yellow- brown. It was later discovered that the yellow and brown hues were actually caused by the heating of the stones as a result of the bombardment. By the 1950s and 1960s cyclotrons and LINACs (linear accelerators) were used to commercially produce colored diamonds. Color penetration tended to be shallow and often in characteristic patterns, and, while blue and green stones were the norm, occasional unexpected hues were encountered.
Later, nuclear reactors were used for treatment, but the high-energy particles tended to zip right through the diamonds; the secondary electrons they generated, however, caused structural defect colors, just like a LINAC. Blue and green hues were the typical result, and these colors were shifted to yellows and browns by heating. It was even discovered that heating a type IA diamond under high pressure could turn it bright yellow, and that a tinge of yellow could result even from the heat generated during diamond polishing!
Heating an irradiated diamond “anneals” the material and changes the color by altering the light absorption of the defect color center created by high-energy bombardment. Heat can even eliminate coloration, in the case of the yellow hue in stones containing nitrogen. The GE Company obtained patents during the 1970s for a process of removing the yellow color in diamonds. But in the 1990s the company found that heating under very high pressure worked even better and was also capable of actually creating diamond crystals from carbon in various forms. (One famously reported GE result was making diamond from peanut butter!). This method of “growing” diamond was labeled HPHT (high-pressure high-temperature).
HPHT is now a widely used commercial process and is employed by companies in Russia, Sweden, Asia, and the U.S. (notably by Chatham, and Gemesis in Sarasota, Florida). Diamonds can be produced in a wide range of colors, and these colors can be further modified by irradiation and heating. The combination of HPHT, irradiation, and heat is now capable of generating a diamond in virtually ANY DESIRED COLOR. This creates an ongoing challenge for gem laboratories. The detection problems that have plagued other gemstones now apply to diamond as well. The urgency of this challenge forced the major testing labs to develop methods for detecting diamond synthetics and treatments, and so far they have kept up with all new technologies. Here is a general overview of the products created by HPHT.
Diamonds generally emerge from the equipment as yellow/brown stones (type IB) and sometimes yellow or greenish or orangey (type IIA). The few colorless stones are type IIA and can have a light gray or blue tone, especially if some boron is added. If these original crystals are irradiated, greenish, yellowish-green, or bluish-green stones tend to emerge. If the original stones are irradiated and then annealed at high temperature, type IB stones can turn pink, orangey, brownish, or purplish-pink. Type IB and some IA stones can turn red, purple, and orangey-red. Lucent Diamonds (Denver) is marketing “Imperial Red” diamonds that result from the three-step process (HPHT annealing + irradiation + low temperature annealing). Gemesis is mass-producing yellow stones, and Chatham is marketing synthetics in yellow, blue, and pink hues. The manufacturer employed by Chatham seems to be using a slightly modified HPHT process, because the stones are less saturated and more evenly colored, making them harder to identify as synthetic.
The HPHT method can produce diamonds in reasonably short times only if a flux is used; carbon is dissolved in the flux and precipitates out as diamond crystals. The fluxes used are metals: iron, nickel and cobalt. These metals quite often appear as microscopic inclusions within the diamonds and are an extremely useful means of identification. Iron can sometimes be present in sufficient quantity to make the stones magnetic! The HPHT method generates very well formed diamond crystals. Small crystals (up to 3 carats) are produced in a matter of a few days! Larger stones are possible but take a longer time to grow.
The limitation on crystal size in HPHT led to a search for other methods of diamond synthesis. Thin film research as early as the 1940s and 1950s, especially in Russia, led eventually to a process called CVD (chemical vapor deposition). Only a few years ago, Apollo Diamond (Boston) started growing and marketing gem-sized CVD diamonds. Their process decomposes methane gas at high temperature, releasing carbon, which then deposits onto a substrate at relatively low temperature and pressure. Gem sizes from CVD crystals range from melee [less than ¼ carat] up to about ¾ carat, but one great advantage versus HPHT is that CVD diamonds can easily be made colorless (although dark brown and fancy orange and pink stones are also produced). And the process can be scaled up without enormous cost. The presence of metallic particles in a diamond is the easiest situation – proof positive of HPHT origin. Anomalous birefringence, a common feature in natural diamonds, is observed only along growth sectors in HPHT synthetics.
Since manufactured diamonds are typically cubic (natural stones are overwhelmingly octahedral), cubic growth patterns visible in ultraviolet light are definitive for synthetic origin. Other properties that have become increasingly important include spectroscopy (especially in the infrared), cathodoluminescence (CL, fluorescence under a beam of electrons), and fluorescence under ultraviolet (UV) light (both SW, short wave, and LW, long wave).
The DTC (Diamond Trading Company, an arm of DeBeers) in the mid 1990s developed several instruments designed to assist the jewelry trade in identifying synthetic diamonds. “DiamondSure” measures absorption of visible and UV light and spits out “pass” (the stone is natural) or “refer” (more tests are needed). DTC claims the machine identifies 100% of HPHT synthetics and only 1- 2% of natural diamonds are “referred” for more testing. “DiamondView” does photoluminescence (PL) imaging using short wave UV and records a digital image of the surface luminescence. “DiamondPlus” does high sensitivity measurements using lasers and spectrometers, with stones held in liquid nitrogen. (Spectra become much sharper at ultra-low temperatures).
The gemological literature is filling up with articles and reports of new testing methods and new processes. CVD methods are being used to apply diamond coatings to cubic zirconia and other stones. New combinations of irradiation and heat are constantly being tried on synthetics made with all the known methods. It is difficult even to keep all the test results straight in your head when doing an evaluation.
Detection of CVD, HPHT, and other synthetic diamonds, one of the biggest challenges to the gemological world, relies on measuring a variety of properties, few of which individually can be used to make a definitive ruling. Moreover, the required equipment is typically complex and expensive and far beyond the reach of jewelry stones and small laboratories. Realistically, unambiguous certification of a diamond as natural versus synthetic can only be properly done by major testing laboratories. The cost of analysis is high enough to prevent all but a relative handful of polished diamonds to be examined. This means that perhaps 99.999% of ALL polished diamonds will never be certified. Bulk screening has been offered as a potential solution, but some screening methods only identify gems that might not be natural and therefore will require further testing. Newer technologies are being developed that might offer somewhat more reliable confirmation. The major issue is not “can you identify” a manufactured diamond. The issue is “will you do the proper testing.” This is a function of cost, which may turn out to be prohibitive for all but larger and more expensive stones. The disparity offers to unscrupulous traders the opportunity and incentive to add synthetic polished diamonds to parcels of natural stones, relying on their identical appearance and properties to avoid detection. It is possible that, in the future, many small diamonds will be sold with no guarantee that they originated in a mine and not a laboratory or factory.
NOTE: HPHT has been used to convert low quality amber into stunning chunks of transparent right red and green material, totally unlike any amber found in nature. The process even preserves some of the insect remains that are typically found in amber from certain localities as well as “stress formations” that are routinely used to identify natural amber. It seems likely that additional uses will be found for HPHT in the future for the treatment of other gem materials.