By Jeff Miller found at RAPAPORT...  07/06/07

The following article was adapted from Gems & Gemology magazine by Russell Shor.

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.

Some of the samples showed scratching, as on this orange-coated diamond. Photomicrograph by Shane F. McClure/©GIA

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. 

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.