ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Brittle, UnevenArthur Thomas, Gemstones (2009), Brittle, Uneven
SiO 2(+Al 2O 3)Walter Schumann , Gemstones of the world (2001)
(Mg,Fe)Al 3(BO 4)(SiO 4)OUlrich Henn and Claudio C.