A number of new diamond treatments have appeared in the last decade. Between fracture filling and high pressure/high temperature (HPHT) treatment, one sometimes forgets about the older, once-popular treatment of diamond coating. This practice was discussed by early GIA gemologist Eunice Miles in her groundbreaking article, “Diamond-coating techniques and methods of detection,” in the Winter 1962–63 issue of Gems & Gemology (pp. 355–364).

Diamonds may be coated to deepen their color (in the case of fancy colors) or to diminish the amount of apparent color (in the D–Z range). Since the majority of gem diamonds are type Ia, with varying degrees of yellow due to nitrogen impurities, historically the most common coating technique has been to paint a thin blue film on diamonds of a near-colorless or lower color grade. This type of coating is typically confined to the crown and pavilion facets close to and on the girdle edge, though it may completely cover the diamond’s surface. Since blue and yellow are complimentary colors, they appear to cancel each other out to produce a more colorless appearance. (Ever wonder why the flutes in most parcel papers are light blue?) It has been almost 20 years since a Lab Note on a coated diamond was last published (see Summer 1984 Lab Notes, p. 107). While, as then, it is true that only a few coated diamonds are submitted to the lab each year, the practice has not completely disappeared.

Recently, two round brilliant diamonds (1.09 and 3.03 ct) were submitted, by separate clients, to the East Coast laboratory. In the course of grading, the presence of a coating in the form of pale blue spots was observed while the diamonds were being examined with magnification. The figure shows the coating on the 3.03 ct stone, which displays what Eunice Miles reported in 1962 as a faint blue “flux” that is characteristic of this treatment. Although GIA does not grade coated diamonds, for the purposes of this report we determined that the color of the smaller stone was equivalent to the “Q–R” range, while the larger appeared to be “M.” In many instances, the lab has observed this treatment with diamonds in the near-colorless range, but over the years staff members have seen it applied to diamonds with color grades through light yellow. Another clue to this type of treatment is often a “grayish” overall body color evident when these stones are observed table-down in a standard color grading environment.

Upon detection of the treatment, these two diamonds were returned to the clients to remove the coating before grading could continue. Since they were not resubmitted as of this writing, we do not know their true colors.

Joshua Sheby
GIA Gem Laboratory, New York