RAPAPORT... The colors of gem diamonds are caused by atomic-level defects in the crystal lattice. These defects can be modified or reorganized by various treatments. Annealing of some natural diamonds under high pressure-high temperature (HPHT) conditions is a very effective, and now a well-developed, diamond treatment method that can substantially improve the color appearance of off-color stones. Depending upon the chemistry and defect configuration in the starting materials, some diamonds can be decolorized, whereas others become attractive fancy colored diamonds after treatment. HPHT treatment was introduced to the gem world about a decade ago and is now performed routinely around the world. It has developed into one of the most important treatment techniques for diamond. Despite the large number of natural diamonds of various types that have been processed, new categories of treated products are still being introduced to the trade. Recently, many intensely colored yellow-orange HPHT-treated diamonds of a type not previously seen have shown up in the jewelry market.
GEMOLOGICAL FEATURES
Unlike many other HPHT-treated diamonds, these new treated diamonds displayed intense and attractive yellow-orange colors (see figure 1). Most were graded as Fancy Deep yellow, yellow-orange, or orange-yellow colors by the Gemological Institute of America (GIA) Laboratory. A slight brownish hue was observed in a few stones, but the majority showed a pure yellow-orange color that was evenly distributed. In contrast to the more common "green transmitter" HPHT-treated fancy colored diamonds, which usually show a greenish hue of varying intensity, the treated diamonds from this new group showed no such green hue. The stones varied from less than 1 carat to over 7 carats in size.
One of the most notable features of this new product is the common occurrence of symmetrical clouds of various shapes (see figure 2). The clouds usually consist of dense, whitish masses of pinpoint particles that are concentrated in the middle of the stones and often form square shapes or appear as "petals" stretching throughout the diamond.
Graphitization is common in the treated diamonds and usually consists of very small graphite flakes that may show clear hexagonal shapes (see figure 3). The graphite flakes are often linearly distributed in arrays that follow directions of crystal growth. Such alignments of graphite flakes are rarely observed in other HPHT-treated or natural diamonds. When exposed to ultraviolet radiation (UV) using conventional gemological long-wave (LW) or short-wave (SW) UV lamps, these stones consistently showed either very weak orange fluorescence or none at all.
Spectroscopic analysis revealed that these new HPHT-treated yellow-orange diamonds are all type Ia with high concentrations of nitrogen impurities. Extremely strong infrared absorption peaks of 1405 and 3107 cm-1 indicated that high concentrations of hydrogen impurities were also present. With a desktop gemological spectroscope, these stones occasionally showed only a weak 415-nanometer (nm) band, but usually displayed no clear absorption bands. Absorption spectroscopy in the UV-visible-NIR (near infrared) range showed a smooth and rapid increase in absorption from about 550 nm toward the blue end of the spectrum. This absorption feature and the resulting intense yellow-orange color are caused by the presence of isolated nitrogen atoms in the diamond lattice that are produced by HPHT treatment. In contrast, the rarely occurring natural color diamonds in this grade range most often have a strong yellow reaction when exposed to LW UV radiation and a weaker reaction to SW. The spectroscopic features that one would observe through a hand spectroscope are also notably different. A weak 415 nm line may be present, but it is usually accompanied by a band at approximately 425 nm and a broad band absorption centered around 500 nm. The naturally colored diamonds also do not typically have the cloud inclusions that were described above.
STARTING MATERIALS
The occurrence of clouds and consistently high concentrations of hydrogen in the new yellow-orange treated diamonds strongly indicated that the starting materials were hydrogen-rich (H-rich) type Ia diamonds. Natural H-rich, type Ia diamonds are commonly gray or brownish-greenish yellow in color due mostly to hydrogen-related defects. Unlike most lower-hydrogen brown diamonds that are subjected to HPHT treatment, plastic deformation in these H-rich diamonds is very weakly developed, if at all. In order to reproduce the treatment, many type Ia brown diamonds, including some H-rich stones, were selected for HPHT treatment. Consequently, most of the plastically deformed, lower-hydrogen brown diamonds turned yellow-green after treatment due to the formation of a complex N-related (nitrogen-related) defect. The H-rich stones turned intense yellow-orange due to the formation of simple, isolated nitrogen defects. The more complex defects that commonly create a greenish hue in HPHT-treated diamonds were not formed in the H-rich diamonds (see figure 4).
IDENTIFICATION
When intensely colored yellow-orange diamonds with symmetrical clouds and very weak UV fluorescence are encountered, a gemologist should proceed with extreme caution. Small, well-aligned graphite flakes are good indicators of HPHT treatment, but are not diagnostic. Some natural diamonds with similar gemological features have been documented. In many cases, absorption and photoluminescence spectroscopy are necessary for a confident identification. The GIA Laboratory is constantly focused on diamond treatments and methods for their identification in order to assure the trade that all treated diamonds can be properly identified.