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‘Peculiar’ Coloring Helps GIA Spot Treatment

April 08, 2021

RAPAPORT... The Gemological Institute of America (GIA) has identified a fancy-color diamond containing rare lines of blue, positing that the stone was originally brown before it underwent treatment.

The round brilliant, 1.50-carat, fancy-intense-green-yellow diamond contained blue “graining,” an unusual phenomenon, according to researchers from the GIA in Carlsbad, California. While fancy-color diamonds often have an uneven hue, linear graining is usually brown, pink or yellow, it noted in the Winter 2020 edition of Gems & Gemology, the GIA’s academic journal, which it released this week.

“Initially, this stone likely had a brown color,” wrote Britni LeCroy, staff gemologist, and Virginia Schwartz, supervisor of diamond identification. “HPHT [High Pressure-High Temperature] treatment then influenced the body color by destroying the initial brown color before creating most of the yellow component, as well as green luminescence.”

The stone, which turned up recently at the Carlsbad laboratory, showed evidence of both HPHT treatment and artificial irradiation, the article continued.

The distribution of color around the stone was unusual, as people who treat polished diamonds tend to do so on the pavilion, not the crown, as was the case here. Magnification using diffused light showed the “peculiar” blue lines, in addition to yellow and brown graining, LeCroy and Schwartz observed.

Graining is rarely the cause of blue coloring in treated or untreated diamonds; rather, this almost always comes from boron atoms getting trapped in the diamond lattice, or from exposure to radiation. The team did not find any boron impurities, leading to the assumption that artificial radiation treatment was the cause of the blueness.

Image: The treated diamond with blue and dark-yellow graining. (Britni LeCroy/GIA)

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