Lab Notes Gems & Gemology, Summer 2018, Vol. 54, No. 2

Large HPHT-Treated Blue Type IIb Diamond

HPHT-treated blue diamond (left) with ragged-looking graphitization around an inclusion (center) and high-order interference colors with crossed-polarizing filters (right).
Figure 1. This 17.09 ct HPHT-treated blue diamond (left) has ragged-looking graphitization around an inclusion (center, field of view 0.76 mm) and high-order interference colors when viewed with crossed polarizing filters (right, field of view 7.19 mm). Photos by Jian Xin (Jae) Liao (left) and Evan Smith (center and right).

Over the past two decades, high-pressure, high-temperature (HPHT) treatment has emerged as a prominent technology for changing the color of diamonds. It is best known for removing brown coloration from natural type IIa material to yield colorless or, rarely, pink diamonds. Similarly, HPHT treatment can be used to reduce the brown color component of gray or brownish natural type IIb (boron-bearing) diamonds, thereby increasing the underlying blue color. Improvements to equipment and methods continue to push the capabilities of HPHT processing.

GIA’s New York lab recently encountered a notably large HPHT-treated Fancy Intense blue oval-cut diamond (figure 1, left). At 17.09 ct, this is the largest HPHT-treated blue diamond graded by GIA to date. For comparison, the average size of most submitted HPHT-processed blue diamonds lies in the 1 to 5 ct range. HPHT treatment can be challenging to detect with standard gemological tools, but in this case the ragged-looking graphitization around an inclusion and the appearance of moderate to high-order interference colors under crossed polarizers were helpful clues (figure 1, center and right). Advanced testing methods (e.g., infrared spectroscopy and photoluminescence spectroscopy) confirmed that the stone was HPHT treated.

Color zonation in HPHT-treated blue diamond.
Figure 2. Color zonation (left) becomes more noticeable when viewed through the pavilion. The stronger blue-colored region corresponds with bright greenish blue fluorescence in the DiamondView (right, field of view 5.8 mm). Note that the DiamondView image is taken from the opposite side of the diamond. Images by Evan Smith.

Additionally, this diamond had a striking blue color zonation (figure 2, left) that is thought to be unrelated to HPHT treatment. A region of strong blue color is separated from an almost colorless region by a sharp division with octahedral plane {111} orientation. Color zonation is not uncommon in natural type IIb diamonds, though it is seldom so sharp and planar (J.M. King et al., “Characterizing natural-color type IIb blue diamonds,” Winter 1998 G&G, pp. 246–268). The stronger blue region likely contains more uncompensated boron. It also exhibits more intense greenish blue fluorescence/phosphorescence in DiamondView images (figure 2, right). The color boundary might reflect a change in defect incorporation during diamond growth.

Evan M. Smith, is a research scientist, Paul Johnson is an analytics manager, and Kyaw Soe Moe is a research associate at GIA in New York.