ConchoidalArthur Thomas, Gemstones (2009)
ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Tourmaline is a series of several different minerals with unique chemical formulas.
SiO 2(+Al 2O 3)Walter Schumann , Gemstones of the world (2001)