Kornerupine and Prismatine rare green gemstones: optical axis and pleochroism in Embilipitiya?
How can collectors verify faceted dark green Kornerupine from tourmaline or enstatite using a handheld polariscope and dichroscope in southern gem pits?
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Kornerupine (a complex magnesium-aluminum borosilicate) and its boron-rich analog Prismatine are rare collector gemstones found in the metamorphic gravels of Embilipitiya and Kolonne. They are regularly mistaken for chrome tourmaline, actinolite, or enstatite. Under a calcite dichroscope, Kornerupine exhibits intense trichroism: emerald green, reddish-brown, and pale yellowish-brown. It is biaxial negative with a refractive index of 1.665 to 1.682 and birefringence of 0.013-0.017. Tourmaline is uniaxial negative with higher birefringence (0.020+), whereas enstatite is biaxial positive with distinct absorption spectra at 505nm.
Rare cat's-eye Kornerupine displays sharp silvery chatoyancy when cut as a cabochon, created by parallel inclusions of hollow tubular channels.
Stones over 3 carats with pure chrome-green coloration are rare and command high prices at specialized gem auctions in Colombo.
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