ConchoidalAnthony et al, Handbook of mineralogy (2001), ConchoidalAnthony et al, Handbook of mineralogy (2001), Brittle, Conchoidal
Gems, Sixth Edition (2006)
Al 4(Ta,Nb) 3(O,OH,F) 14Ulrich Henn and Claudio C.
SiO2 (with a high level of nickel impurities )