tourmaline science definition

Any of several minerals having the general chemical formula (Na,Ca)(Mg,Fe,Al,Li)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄. Tourmaline occurs in many different translucent colors, usually in crystals shaped like 3-, 6-, or 9-sided prisms. It occurs in igneous and metamorphic rocks, especially in pegmatites.