Bismutotantalite: Gemstone Information

Bismutotantalite is Bi(Ta,Nb)O4 and forms stout prismatic crystals or masses of the orthorhombic system, usually opaque and coloured light brown to smoke-grey with adamantine or submetallic lustre, hardness 5,SG 8.15–8.89.Facetable material is rare but the RI for the alpha, beta and gamma rays is 2.388–2.395, 2.403–2.408 and 2.426–2.428, biaxial positive with birefringence 0.040. Bismutotantalite occurs in granite pegmatites and as water-rounded pebbles. Sources of ornamental material are the Muiane mines, Mozambique and Acari, Brazil.