ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Brittle, UnevenArthur Thomas, Gemstones (2009), Brittle, Uneven
Al 2Be 3Si 6O 18Martin H.
(Mg,Fe)Al 3(BO 4)(SiO 4)OUlrich Henn and Claudio C.