Black tourmaline in matrix

Black, opaque, vitreous-to-resinous luster with elongated striations and likely prismatic crystal form; Mohs hardness 7–7.5, poor cleavage, specific gravity about 3.1–3.3. The pale host appears to be quartz or feldspar matrix.

Formed in boron-rich hydrothermal veins and granitic pegmatites as magma cooled, commonly during the Precambrian to Phanerozoic. Tourmaline crystals grow in cavities and along fractures, often attached to quartz or feldspar.

Used as a collector mineral and occasionally as a gemstone; large transparent tourmalines are cut for jewelry, while black schorl is mainly ornamental and educational.

Schorl is the most common tourmaline species and is strongly piezoelectric and pyroelectric: electrical charges develop when it is heated or compressed.

Identify it by its opaque black color, lengthwise striations, glassy luster, and hardness sufficient to scratch glass; distinguish it from hornblende by its sharply striated, often triangular-rounded prism. Common sources include Brazil, Afghanistan, Pakistan, Madagascar, and the United States, especially Maine and California.