Sinhalite (MgAlBO4) rare collector gemstone: identifying pleochroism and RI properties?
4 Answers from travellers
Sinhalite—first identified in Sri Lanka as a distinct borate mineral species rather than a brown peridot or chrysoberyl—presents specific diagnostic optical constants. Under a gemmological refractometer, Sinhalite is biaxial negative with refractive indices of alpha = 1.668, beta = 1.698, and gamma = 1.706, yielding a high birefringence of 0.038. This high birefringence produces doubled facet junctions through the table facet. Under a calcite dichroscope, it displays trichroism: pale brown, greenish-brown, and dark brown. Chrysoberyl has a higher refractive index (1.74-1.75), whereas brown tourmaline is uniaxial negative with lower RIs (1.62-1.64).
Sinhalite rough mined from alluvial gravels in Ratnapura often resembles water-worn dark olive or brownish pebbles; check with specific gravity liquids to screen stones rapidly.
Clean, transparent Sinhalite faceted over 5 carats is prized by mineralogical museums and private collectors as an exotic collector stone.
Ensure that export paperwork issued by the NGJA explicitly states "Sinhalite" rather than generic "mixed corundum" to avoid valuation disputes.
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