Conchoidal, Conchoidal, Gemstones of the world (2001)
UnevenArthur Thomas, Gemstones (2009) Tough, UnevenArthur Thomas, Gemstones (2009) Tough, Brittle, Uneven
SiO 2 · nH 2OWalter Schumann , Gemstones of the world (2001)
Mn 8Si 6O 15(OH,Cl) 10Ulrich Henn and Claudio C.