Uneven, Uneven, ConchoidalAnthony et al, Handbook of mineralogy (2001), Brittle, Conchoidal
Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
LiAl(PO 4)(OH,F)Walter Schumann , Gemstones of the world (2001) More from other references
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.