Treated Pink Diamond with HPHT Synthetic Growth Structure
HPHT synthetic diamonds are grown using high pressure and high temperature but with a much higher growth rate than natural diamond. Consequently, their growth structures are different. GIA’s New York lab recently examined a multi-step treated diamond with a growth structure similar to that of HPHT synthetics. The 1.62 ct Fancy pink type IIa round brilliant seen in figure 1 showed spectral characteristics suggestive of HPHT treatment, irradiation, and annealing. This diamond was internally clean, except for the presence of strong, colorless internal graining (figure 2). Tatami and banded strains with strong interference colors could be seen under cross-polarized light (again, see figure 2). Based on these microscopic features, we concluded that this was a natural diamond.
DiamondView imaging of the stone showed cuboctahedral growth with dark sectors and pink coloration (figure 3). This crystal habit is usually observed in HPHT-grown synthetic diamonds. As seen in figure 3 (left), the cubic {100} sector can be located in the pavilion, along with octahedral {111} sectors. In order to understand more about this growth, we compared it with a type IIa 0.27 ct HPHT synthetic, which was treated post-growth to induce a Fancy Intense orangy pink color (figure 3, right). Both specimens showed similar fluorescence patterns; however, the natural diamond’s greenish blue phosphorescence was evenly distributed, as seen in the middle image of figure 3. This is unlike the dark images yielded from type IIa pink HPHT synthetics, which are not phosphorescent.
Although most natural diamonds show octahedral growth structure in DiamondView images, some natural gem-quality colorless and yellow diamonds may show cuboctahedral growth (Winter 2010 Lab Notes, pp. 298–299; Winter 2011 Lab Notes, p. 310; Spring 2013 Lab Notes pp. 45–46). Some non-gem-quality diamonds possess cuboctahedral form (D.G. Pearson et al., “Orogenic ultramafic rocks of UHP (diamond facies) origin,” in R.G. Coleman and X. Wang, Eds., Ultrahigh Pressure Metamorphism, Cambridge University Press, 1995, pp. 456–510). Growth rate, pressure, and temperature of geological environment control the habit of a diamond crystal. A slow growth rate causes octahedral crystal growth, but a high growth rate can facilitate cubic {100} and octahedral {111} sectors developing simultaneously. As a result, cuboctahedral habit can be formed naturally.
While the DiamondView is very reliable at revealing the growth structure of natural vs. synthetic diamond, it is important to correctly interpret all identifying characteristics. When cuboctahedral growth structure is observed, one should look for other features to support the origin. Microscopic features and spectral characteristics are useful for this purpose.
