Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Uneven, Irregular, Management Team (2012)
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.
(Mg,Fe2+ ) 3Si 2O 5(OH) 4Anthony et al , Handbook of mineralogy (2001)