By Ashley Bailey
, Monday, July 09, 2007 12:00 AM
From GIA, Carlsbad, Calif.
Adapted from Gems & Gemology by Russell Shor © 2007 Gemological Institute of America. All rights reserved
With the advent of HPHT treatments and sophisticated irradiation techniques, the age-old practice of coating diamonds to enhance their color would seem to be a thing of the past. However, the GIA Laboratory has seen an increase in coated diamonds submitted in recent months, which prompted researchers to investigate a new commercial coating that is now in the diamond market.
Serenity Technologies of Temecula, California, has created a silica coating that produces natural-looking fancy colors, including blue, green, orange, pink, purple-pink, and yellow. This coating is reasonably durable (though not considered permanent), but it can be readily identified by standard testing.
The diamonds examined in this study were coated with a silicate material less than 60 nanometers thick, with other substances added to achieve different colors. While Serenity Technologies did not reveal the nature of the additives, the researchers’ findings point to gold and silver added to create pink and blue, silver to create yellow, and iron to create orange.
GIA researchers Andy H. Shen, Wuyi Wang, Matthew S. Hall, Shane F. McClure, James E. Shigley, and Thomas M. Moses were joined by Steven Novak of Evans Research Group in conducting the study. The group examined 102 diamonds of various colors from dealers whose goods had been coated by Serenity. They subjected the diamonds to spectroscopic analysis, chemical analysis, and microscopic observation under various lighting conditions, followed by a series of durability tests.
Many of the diamonds examined were very small, 0.01−0.03 ct, though some blue, orange, and pink goods were larger. All but the yellows showed strong colors, equivalent to Fancy Intense and Fancy Vivid grades for natural-color diamonds. The yellow diamonds showed slight tinges of brown. In addition, Serenity agreed to treat six larger, near-colorless diamonds (0.34–0.40 ct) supplied by GIA to determine the differences in their gemological and spectroscopic properties after treatment. These ranged in color from Fancy Light yellow to Fancy Deep purple-pink.
The gemological examinations found that the colors appeared evenly distributed when the diamonds were examined face up. However, typically the coating became visible as a colored film when the researchers looked at the diamonds through the pavilion facets in diffused, reflected light. It was particularly strong on the green-coated diamonds. Some of the coatings showed scratches, colorless spots, and small stains.
With the coated pink diamonds, there was none of the pink graining typically found in natural-color pinks. In other colors, many common internal features, such as the high clarity of type IIb blues and the patchy color distribution in natural orange-yellows, were not seen in these samples. All were generally lower-clarity stones.
The treatments had little effect on fluorescence. After processing, the six stones fluoresced the same color to both short and long-wave ultraviolet radiation, but the intensity to long-wave UV was weaker in two samples.
No changes were observed in the infrared spectra of the six diamonds analyzed both before and after coating. The ultraviolet-visible-near infrared spectra of the coated samples were different from those of their natural-color counterparts. The chemical composition of the coatings used to produce the different colors was determined by secondary ion mass spectroscopy (SIMS).
Durability tests were conducted to determine how well the coatings could withstand jewelry setting and repair processes, as well as the wear and tear of everyday exposure to common chemicals.
The coatings in all colors withstood heating up to 500 degrees Celsius but showed a change in color or cloudiness above 600 degrees. However, touching the stone with a polishing wheel after the setting process damaged all of the colors tested. Similarly, the act of soldering prongs in the retipping process caused color changes in most of the goods tested.
The coatings were fairly resistant to jewelry-cleaning operations, including steam cleaning and pickling, but immersion in sulfuric acid for 30 minutes removed the coatings entirely on six of the seven stones. Household chemicals such as detergent and alcohol did not affect the coatings, but two days’ immersion in undiluted bleach caused color fading in blue, green, and yellow samples. The bleach did not affect the orange and pink stones. The coatings were less resistant to scratching. A stainless steel needle point did not damage the material, but common abrasives such as sandpaper and powdered cleanser scratched the coatings of the colors tested (blue, orange, and pink).
The GIA Laboratory will provide an identification report for such stones, noting that the diamond is “surface coated” and indicating carat weight, cutting style and measurements, but it will not issue a color or clarity grade because coatings are not considered permanent.
Figure 1. A variety of colors can be produced on polished diamonds (here, 0.01–0.70 ct) by a new coating technique from Serenity Technologies, Inc. Composite photo by Jessica Arditi and Jian Xin Liao/©GIA.
Figure 2. These six diamonds (0.34–0.40 ct) were colorless to near-colorless before being subjected to the Serenity coating process. Composite photo by Jian Xin Liao/©GIA.
Figure 3. Some of the samples showed scratching, as on this orange-coated diamond. Photomicrograph by Shane F. McClure/©GIA; magnified 22×.
Figure 4. Also visible on some stones were spots or patches of uncoated areas, as seen with reflected light on this blue-coated diamond. Photomicrograph by John I. Koivula/©GIA; magnified 10×.
Figure 5. This vivid orange-coated diamond (left) changed to an intense yellowish orange (right) when exposed to a high-temperature solder during retipping. Photos by Jian Xin Liao and Robison McMurtry/©GIA.