Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Uneven, Brittle, Uneven, Brittle, Uneven
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.
(K,Na)Ca4Si8O20(F,OH)•8(H2O)