corundum, inclusions.5, Thailand.12, gems

Sapphirine inclusions are identified in a rough ruby from Bo Rai, Trat Province, Thailand, in what the authors report as the first documented occurrence of this mineral as an inclusion in any gem material. The host is a purplish-red, tabular, etched crystal weighing 0.91 ct, collected by Peter C. Keller during investigations of the local ruby mines. Three dark greenish-blue grains occur within the ruby, the largest exposed where the crystal had broken across its centre. Their unusual appearance prompted examination after conventional gemmological testing confirmed the identity of the host. Microscopy showed rounded, anhedral grains, suggesting that they formed before their incorporation into the ruby. Because the inclusions were very dark, an 80 W pinpoint fibre-optic illuminator was required to transmit sufficient light through the exposed grain for optical examination. It proved birefringent and displayed green and deep blue pleochroic colours, initially suggesting sapphire. To test this interpretation, the exposed inclusion was scraped with a fine diamond-tipped tool to obtain a minute quantity of powder for X-ray diffraction. The sample was mounted in a Debye–Scherrer powder camera and exposed for 4.5 hours using nickel-filtered copper radiation generated at 46 kV and 26 mA. Its diffraction pattern differed clearly from the corundum reference pattern, excluding sapphire. Interplanar spacings were measured with a Nies overlay scale corrected for film shrinkage, and diffraction-line intensities were estimated visually. Comparison with reference data produced an exact match with monoclinic sapphirine, a magnesium aluminium silicate represented by the formula (Mg,Al)₈(Al,Si)₆O₂₀. Although sapphirine had previously been described as a faceted gemstone, its occurrence enclosed within another gem was considered a new observation. The authors emphasise the ease with which small sapphirine grains can be mistaken for sapphire because of their similar appearance, despite substantial chemical and physical differences. In this specimen, colour, pleochroism and association with ruby all favoured an initially plausible but incorrect identification. The study demonstrates the importance of analytical confirmation when identifying unfamiliar inclusions whose optical appearance resembles a more common mineral.