Thomsonite-(Ca): Gemstones Information
Thomsonite, NaCa2Al5Si5O206H2O, is a common member of the zeolite group of minerals and forms transparent to translucent orthorhombic pseudotetragonal thin or bladed prismatic crystals with one direction of perfect cleavage and a range of pale colours, notably yellow, pink,greenish or grey. Fibrous masses are more common and occur as radiating spherical or columnar aggregates. The hardness is 5–5.5 and the SG 2.25–2.40. The RI for the alpha, beta and gamma rays is 1.497–1.530,1.513–1.533 and 1.518–1.544, biaxial positive with birefringence 0.021.Thomsonite occurs in amygdules and fractures in mafic igneous rocks,most commonly basalts. The eye-like material usually comes from Isle Royale, Michigan, USA.