Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Handbook of mineralogy (2001), Difficult, Fibrous
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.
Ca 6Si 6O 17(OH) 2Michael OâDonoghue , Gems, Sixth Edition (2006)