Thaumasite: Gemstones Information

Thaumasite is Ca3Si(CO3)(SO4)(OH)612H2O and forms transparent to translucent white, yellowish, pink or brown acicular prismatic crystals of the hexagonal system or compact masses. The hardness is 3.5 and the SG about 1.90. The RI for the ordinary and extraordinary rays is 1.498–1.507 and 1.458–1.470, uniaxial negative with birefringence 0.036. Thaumasite occurs as a very late-stage mineral in some sulphide ore deposits. As oneof the zeolite mineral group, it associates with other group members.Yellow material from the Wessels and N’Chwaning mines,Kuruman,Cape Province, South Africa, has been faceted.