Kornerupine: Gemstone Information
Kornerupine is (Mg,Fe)(Al,Mg,Fe)9(Si,Al,B)5(O,OH,F)22 and occurs as transparent prismatic crystals of the orthorhombic system with hardness 6–7 and a good cleavage in one direction. Colours include a characteristic dark green inclining to brown and a rare and attractive bright vanadium green. The SG is 3.28–3.35 and the RI for the alpha,beta and gamma rays is 1.660–1.671, 1.673–1.683, 1.674–1.684, biaxial negative (may be pseudouniaxial) with birefringence 0.013–0.017.Pleochroic colours are brown to greenish brown. A weak absorption band may be seen at 503 nm. Chatoyant stones show a bright eye against a dark green background. Vanadium-coloured stones are a very attractive bright apple-green and were rare at the time of writing.Kornerupine occurs in boron-rich volcanic and sedimentary rocks subjected to metamorphism. Most gem material is found in the Sri Lanka gem gravels in the Matale and Ratnapura districts and gem crystals (some a pale blue) also occur at Itrongahy, Betroka, Madagascar. The Myanmar gem gravels have produced greenish brown specimens. The vanadium-green specimens appear to come only (so far) from the Kwale area of Kenya; some may show a yellow fluorescence.