Leucite: Gemstone Information
Leucite is KAlSi2O6 and occurs commonly as transparent to translucent colourless or white euhedral pseudocubic crystals of the tetragonal system.The hardness is 5.5–6 and the SG 2.45–2.50. The RI for the ordinary and extraordinary rays is 1.508 and 1.509, uniaxial positive (may be biaxial) with birefringence very low to measure with gemmological instruments. Some but not all specimens may show a medium to bright orange under LWUV and a bluish colour under X-rays. Leucite occurs with potassium-rich maficand ultramafic lavas, most specimens coming from the Alban Hills, Rome,Italy; this site provides virtually all leucite that can be fashioned.