- any of a group of silicate minerals of complex composition, containing boron, aluminum, etc., usually black but having various colored, transparent varieties used as gems.
- any of a group of hard glassy minerals of variable colour consisting of complex borosilicates of aluminium with quantities of lithium, sodium, calcium, potassium, iron, and magnesium in hexagonal crystalline form: used in optical and electrical equipment and in jewellery
complete silicate of aluminum and boron, 1759, ultimately from Sinhalese toramalli, a general name for cornelian.
- 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.