Sinhalite: Gemstones Information
Sinhalite is MgAlBO4 and is most commonly found as transparent honey yellow to brown grains or pebbles of the orthombic crystal system. The hardness is 6.5–7 and the SG 3.475–3.50. The RI for the alpha, beta and gamma rays is 1.667–1.676, 1.697–1.704 and 1.705–1.712, biaxial negative with birefringence 0.035–0.037. The absorption spectrum shows bands at 493, 475, 463, 452 and 435 nm with general absorption of the violet: the band at 463 nm is not present in the peridot variety of olivine (q.v.) with which there might otherwise be confusion. Sinhalite occurs in boron-rich skarns at the contact of limestones with granite or gneiss; most gem sinhalite is found as alluvial pebbles. One crystal from Myanmar was described in 1958. A transparent golden brown chatoyant variety with a silver eye is reported from the main locality for sinhalite, Sri Lanka.