We publish courtesy of GIA’s G&G eBrief
Author: Wuyi Wang GIA Laboratory, New York
Gem laboratories frequently see colorless to near-colorless HPHT-treated type IIa diamonds, most of them smaller than 5 ct. In recent weeks, however, GIA’s New York lab has examined an increasing number of relatively large HPHT-treated type IIa diamonds, many weighing more than 5 ct and some 10+ ct.
One example is the 18.12 ct diamond at right, which was color graded F. While many HPHT-treated diamonds have no observable internal features except for some graining, this stone contained a tiny hexagonal graphite inclusion surrounded by a tension fracture. The inclusion, which very likely formed due to graphitization of the host diamond, and the tension fracture are good indications that this stone was HPHT treated. Careful spectroscopic analysis provided confirmation.
HPHT annealing involves a higher risk of damaging the diamond than other treatment techniques, so it is somewhat unusual to see it applied to such large stones. It is not clear if this surge in submission of large treated diamonds is a short-term phenomenon or the beginning of a trend. One possible explanation is that more suitable starting materials have become available in the market.