Synthetic Star Sapphire with Hexagonal Zoning
Authors: Gagan Choudhary and Chaman Golecha
(This article was first published in Gems & Gemology, Summer 2007, pp 177-178)
Hexagonal colour/growth zoning is a classic identifying feature for natural corundum. However, at the Gem Testing Laboratory in Jaipur, India, we encountered a synthetic star sapphire (figure 1) with hexagonal zoning. Our initial examination indicated that the sample was a natural sapphire with a diffusion-induced star, as suggested by the wavy appearance of the rays and the presence of “silk” inclusions. The 4.63 ct cabochon fluoresced chalky blue to short-wave UV radiation and showed chromium lines when observed with the desk-model spectroscope.
Figure 1: This 4.63 ct cabochon proved to be a synthetic star sapphire with hexagonal zoning in its core, which initially suggested natural origin
With magnification and immersion, a hexagonal zone was evident in the core of the cabochon when it was viewed from above (figure 2, top). This zone was surrounded by a wavy stress pattern (see figure 2, top and bottom). Also present were fine needles oriented in three directions, which were responsible for the star effect.
Figure 2: A central hexagonal core was visible when the cabochon was observed with magnification and immersion (top). A wavy stress pattern was present in the area surrounding the core (top and bottom). Magnified 20x (top) and 30x (bottom)
When the cabochon was viewed from the back, however, we were surprised to discover numerous tiny whitish pinpoints (likely gas bubbles) arranged in curved clouds (figure 3, left). When the stone was viewed in immersion with diffused illumination, curved colour bands also became apparent (figure 3, right). These features are diagnostic of a flame-fusion synthetic origin.
Figure 3: Curved clouds of whitish pinpoints (apparently gas bubbles) were evident when the base of the cabochon was viewed with magnification (left). With immersion and diffused illumination, curved colour bands were also seen (right); these are conclusive of synthetic origin. Magnified 25x (left) and 20x (right)
The cause of the hexagonal zone is not clear. We speculate that it may have been the result of a crystallographically oriented concentration of silk in the core. Using higher magnification and a strong fibre-optic light, we noted a concentration of the needles in the core as compared to the surrounding area. This was the first time we observed both hexagonal zoning and curved colour bands in a single sample. It provides an important reminder that a gemmologist should avoid making identification without considering all of the evidence presented by a sample. If this stone had been mounted in a closed-back setting, it would have been very difficult to make a correct identification.
All photographs and photomicrographs by Gagan Choudhary