Another Diamond Type Is Susceptible to HPHT
Rare Type IaB diamonds are targeted
|A frenzied race is on at diamond synthesis and treatment facilities around the world to develop high-pressure/high-temperature treatments that improve some diamonds to near colorless. Treaters also are using HPHT methods to cause color changes in natural brown diamonds.
Gem labs are working hard to make sense of the changes and establish new methods of detecting the treatments. The most common diamonds Type Ia appear to have eluded HPHT enhancement to near colorless so far, though they can be turned yellow-green.
Meanwhile, this article reports on a new joint effort that discovered another diamond type that appears susceptible to HPHT enhancement.
The use of high-pressure/ high-temperature treatment to change the color of diamonds raised concerns when it was made public two years ago. Since then, the industry has grown a little more accepting, largely because the treatment works on a limited number of diamond types. A new study finds HPHT can be used on another diamond type. But first, the history.
Since March 1999, General Electric USA has used a proprietary HPHT method to change rare brown Type IIa diamonds to near colorless to colorless. In February 2000, GE said it also could use the process to produce gem-quality yellow to green neon Type Ia, pink Type IIa and blue Type IIb diamonds.
In 1999 NovaDiamond Corp. of Provo, Utah, announced a new HPHT process for changing the color of natural brown Type IaAB diamonds to a more appealing intense yellow to green. This process involves a new generation of HPHT presses, referred to as prismatic presses (Deljanin and Sherman, 2001). Since then the company has learned to change brown Type IIa and IaB diamonds to near colorless and pinkish.
Not all HPHT treatments are recent. In Russia, for example, scientists in Novosibirsk and Moscow have used an HPHT apparatus known as BARS to make synthetic diamonds for almost 10 years. Similar equipment is used now to change natural diamonds to pink, yellow and orange.
HPHT technology raises concerns about the color origin of many natural colored and colorless diamonds, including those that gem labs certified years ago.
At EGL USA in New York City and at the Institut des Materiaux Jean Rouxel, Laboratoire de Physique Cristalline Equipe Gemmologie in Nantes, France, we investigated HPHT treatment of a different type of diamond not reported on before: Type IaB. These are rare diamonds that contain quadruplet nitrogen atoms (constituting four atoms of nitrogen surrounding a vacancy). Scientists call this structure the B aggregate. Because they contain nitrogen, they belong to the more common group of Type Ia diamonds and are classified as Type IaB.
These diamonds are transparent to shortwave ultraviolet light (254nm), much like type IIa diamonds. They have little absorption in the blue part of the visible spectra (unlike regular Type Ias), so they also have some features in common with the IIas.
The majority of natural diamonds mined around the world contain nitrogen as an impurity (replacing carbon and often imparting yellowish color). In fact, 98% of all diamonds are Type Ia. If there is a way to improve the color of diamonds with nitrogen as an impurity through the HPHT process, the whole diamond industry could face major changes. For example, many light yellow cape diamonds could be improved to more intense yellow or orange yellow, and low-nitrogen brownish diamonds could be converted to much lighter colors. EGL USA has seen a few such diamonds already and is studying them. Fancy colors, therefore, seem to be the area most likely to face the major changes we mention.
The goal of EGL USAs research project was to explain the HPHT process in detail and find criteria to separate natural-color, low-nitrogen Type IaB and IIa diamonds from treated ones using classical and advanced gemological instruments such as an optical microscope, UV-VIS-NIR spectrophotometer, infrared spectrometer, 532nm laser (to assess photoluminescence spectra), FT Raman, cold cathode and scanning microscope for hot cathodoluminescence.
Over 11/2 years at EGL USA and IMN, we compared several pure Type IaB HPHT-treated diamonds having low nitrogen content with Type IIa diamonds after processing in the same conditions. These natural diamonds originally were low-quality, fancy to light brown, 0.54 to 3.14 carats. Half were rough crystals, half were polished. Irregularly shaped rough and polished samples came from alluvial deposits in Brazil.
Most showed brown graining, as expected in the vast majority of diamonds regardless of type. These areas have defects that are concentrated during the deformation of the diamond and are highly defective areas compared with the surrounding near-colorless areas.
The diamonds were heated at about 2200&Mac251;C at a pressure of 60 kilobars for less than a minute. Depending on the starting color and amount of nitrogen, color improved to colorless (E-F) to light brown (U-V range). A 1.36-ct. light brown rough sample became bicolored (near colorless and very light yellow) along a cleavage in the middle of the stone. This is the first documented record that the color of IaB diamonds could become much lighter through an HPHT process.
Other factors to note: Small crystals and medium-size feathers put these diamonds in the VS-SI clarity range. Brown graining was less visible after treatment. There was no change in fluorescence.
The brown lamellae, or graining planes, contained a small portion of brown amorphous carbon as the coloring agent (Fritsch and Deljanin, 2001). During HPHT treatment, this amorphous carbon recrystallized on the surrounding well-crystallized regions of the diamond. The final result is as if released vacancies didnt interact with nitrogen, but instead diffused out of the stone.
Just like Type IIas, the Type IaBs became more colorless. In some cases (a 0.54-ct. round and a 3.14-ct. rough), the absorption band at 637nm (N-V defect) was stronger after treatment.
Unlike Type IIa diamonds, no single nitrogen was created after the treatment. Some IaB diamonds showed a weak N3 center after treatment and, as a result, became a light yellow-gray (Deljanin and Fritsch, 2001). Under magnification most showed signs of the very high temperatures (around 2250&Mac251;C) used in an HPHT process. For example, graphitization was intense stones showed black graphite spots and graphite in fractures after treatment. They also took on an overall grayish cast. Surfaces and fractures became frosty and etched. Cathodoluminescence colors became strong whitish blue or violet (a similar reaction occurs in Type IIa treated diamonds).
To identify HPHT-treated diamonds, we rely on a combination of the properties noted above because no single one is sufficient. More work needs to be carried out to understand changes produced by annealing IaB diamonds with medium to high nitrogen concentration.
On the other hand, the risk that HPHT-treated Type IaB diamonds will enter the trade seems minuscule because the results are not as spectacular or dramatic as with the Type IIa diamonds. Also, any effort at commercial application would be hampered by the rarity of Type IaB diamonds.
For more information and handouts about the HPHT process, call EGL USA at (212) 730-7380.
|A Type IaB crystal that was HTHP-treated became bicolored along the cleavage plane in the rough. The left side is colorless and the right side remained yellowish. Photo courtesy of EGL-USA, New York City.
|After HTHP treatment of a faceted Type IaB 0.59-ct. brilliant diamond, note the graphitization of inclusions and etched features in the overall appearance. Photo courtesy of EGL-USA, New York City.
Deljanin, B., Sherman G.E. (2000). Changing the Color of Diamonds: The High Pressure High Temperature Process Explained, Techniques and Identification, EGL Press, October 2000.
Deljanin B., Fritsch E. (2000) A New HPHT Process to Modify the Color of Natural Diamonds. Poster, 11th European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes, Nitrides and Silicon Carbide, Porto, Portugal, Sept. 3-8, 2000.
Deljanin B., Fritsch E. (2001) Characterization of IaB Diamonds Under High Temperature High Pressure Process. Oral presentation, 12th European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes, Nitrides and Silicon Carbide, Budapest, Hungary, Sept. 2-7, 2001.
Fritsch E., Deljanin B. (2001) Les Diamants de Type I Traités à HPHT: Novatek, General Electric, Russes et Suedois. Revue de Gemmologie, Association Française de Gemmologie, Jan./Feb. 2001, Vol. 141/142, pp. 54-58.