Large HPHT-Treated Blue Type IIb Diamond
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.
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.
