Lab Notes Gems & Gemology, Summer 2013, Vol. 49, No. 2

Calcium Fluoride Coating Found on 13 "Fancy" Pink Diamonds

Figure 1. These face-up images of 0.5 ct and 0.45 ct pink round brilliants are representative of the 13 coated pink diamonds submitted. Photo by Jian Xin (Jae) Liao.

Coating gemstones to alter or improve their color is the oldest known enhancement method. Early coatings, particularly on diamond, were easily detected, often visible using a 10× loupe. But advances made after World War II resulted in more durable coatings and films that were subtler and harder to detect.

A common coating was calcium fluoride (CaF2) doped with gold (Au) nanoparticles. CaF2 causes a broad absorption band at about 520 nm, similar to the band at about 550 nm that largely accounts for the pink color of natural diamonds.

Recently submitted to the East Coast laboratory for identification and grading was a group of 13 pink diamonds. The round brilliants ranged from 0.42 to 0.50 ct and had a Fancy pink to Fancy Intense pink color. The above photo shows the patchy appearance characteristic of coated diamonds. Microscopic examination using reflected light revealed obvious coating on the pavilion facets, with patchy colorless areas where it had been removed (figure 2).

Figure 2. Photomicrographs of the pink diamonds from figure 1, taken with reflected light at 80× and 100× magnification, reveal the coating applied to the pavilion facets. Photomicrographs by Paul Johnson.

The UV-visible spectra of all 13 diamonds exhibited a broad band centered at about 520 nm, not at 550 nm as expected. This band is responsible for the pink coloration, and the 520 nm position identified the coating as fluoride.

This example demonstrates that traditional treatments such as coating are still prevalent in the trade, and great care must be taken to identify them. Because coatings are not permanent, all 13 diamonds were issued identification reports disclosing the treatment.