GIA Examines Pink Diamond with ‘Mottled’ Look

January 1, 2026

Suzanne Watkin

The Gemological Institute of America (GIA) recently studied how a natural diamond underwent multiple treatments to achieve its pink color.

The GIA received a round, 1.10-carat, fancy-deep-brownish-orangey-pink diamond at its laboratory in New York for analysis. The stone had a mottled appearance, according to an article in the fall issue of the institute’s quarterly Gems & Gemology journal.

Its findings showed that the diamond had experienced several treatments. These included a combination of High Pressure-High Temperature (HPHT) annealing, irradiation and subsequent moderate heating, to form nitrogen-vacancy (NV) centers and achieve the desired pink hue. Prior to the treatment, the diamond was originally brown, the GIA explained.

When viewed face-up, the stone appeared mottled — which is unusual for a treated pink diamond. Examination under a microscope revealed that the pattern reached from shallow depth to the polished surface.

The visible/near-infrared (Vis-NIR) spectrum uncovered characteristics typical of a diamond treated with laboratory irradiation and heat. The mottled look likely reflected the uneven distribution of these NV centers, which was linked to the nitrogen aggregates. However, the lab did not perform detailed mapping to confirm this.

Image: The pink-diamond examined by the GIA. (Gemological Institute of America)