A blue gemstone pendant was recently submitted to Guild Gem Laboratories for identification. Standard gemological testing identified it as a sapphire, with a refractive index of 1.762–1.770 and birefringence of 0.008. Fourier-transform infrared ( FTIR) spectroscopy combined with microscopic observation confirmed it was a natural sapphire. The UV-Vis spectrum indicated a basalt-related origin owing to relatively high iron content. The FTIR transmitted spectrum also revealed a 3309 series with peaks at 3366, 3309, 3232, and 3185 cm^–1, which are commonly seen in basalt-related sapphire.

Microscopic observation showed several colorless crystals and fluid inclusions under reflective and transmitted light. Using a fiber-optic light, we also observed very interesting scenes of whitish clouds (figure 1) consisting of arrays of hexagonal particles. These particles seemed to be flat and parallel to each other. A distinct uniaxial interference pattern was observed when viewing perpendicular to these particles under the polariscope equipped with a conoscope, which proved that they were parallel to the basal plane (0001) crystalline face of corundum. Further observations revealed that each particle possessed six nearly identical triangular sectors, divided by six arms with hexagonal symmetry. Based on our microscopic observation and estimation, the diameters of the hexagons ranged from approximately 0.06 to 0.13 mm. This pattern was very similar to the trapiche structure seen in some sapphire, ruby, and emerald.

The depth of the inclusions prohibited Raman spectroscopy testing, but we will monitor this phenomenon in other samples for further study in the future.

Figure 1. The regular hexagonal inclusions consisted of six independent units, resembling a trapiche pattern. Photomicrograph and illustration by Yizhi Zhao; field of view 1.70 mm.

(This article was first published in Gems & Gemology, 2019, Vol. 55, No. 3, pp. 440)