Conchoidal, Uneven, ConchoidalArthur Thomas, Gemstones (2009)
Conchoidal, Conchoidal, Gemstones of the world (2001)
(Mg,Fe2+ )Al 2O 4Gemdat.org , Management Team (2012) More from other references
SiO 2 · nH 2OWalter Schumann , Gemstones of the world (2001)