Complex boron and aluminum silicate minerals, tourmalines form slender, three-, six-, or nine-sided prismatic crystals in parallel or radiating groups. Tourmalines develop an electrical charge when heated or deformed, and slabs cut perpendicular to the long axis can polarize light. The best-developed tourmaline crystals are most commonly found in pegmatite. Crystals also are found in limestone altered by granitic intrusions. Tourmaline is the birthstone for October.