Conchoidal, ConchoidalArthur Thomas, Gemstones (2009), Brittle
ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Mg 3Al 2(SiO 4) 3Arthur Thomas , Gemstones (2009)
SiO 2Ulrich Henn and Claudio C.