ConchoidalArthur Thomas, Gemstones (2009)
ConchoidalWalter Schumann, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Brittle, Metallic
Tourmaline is a series of several different minerals with unique chemical formulas.
Be 2BO 3(OH)Ulrich Henn and Claudio C.