A cyclosilicate mineral family with (BO₃) triangular groups and a complex chemical composition. The general formula can be written XY₃Al₆(OH)₄(BO₃)₃(Si₆O₁₈), in which X = Na, Ca, and Y = Al, Fe³⁺, Li, Mg, Mn²⁺. The more common tourmalines are dravite (X,Y = Na,Mg), schorl (X,Y = Na,Fe), uvite (X,Y = Ca,Mg), and elbaite (X,Y = Na,Li). Fluorine commonly substitutes in the hydroxyl position. Tourmaline is a hard (7½ on Mohs scale), varicolored mineral which can be an important semiprecious gemstone.