mineral

Polished black tourmaline (likely schorl)

Schorl, NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

AI-generated identification · may be incorrect

Opaque black, vitreous-to-resinous luster, commonly prismatic with longitudinal striations; Mohs hardness 7–7.5, brittle, no prominent cleavage, and specific gravity about 3.1–3.3. The smooth reflective surface indicates cutting and polishing.

Identified August 31, 2026More mineral →

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Physical properties

Formation & geological history

Forms in boron-rich granitic pegmatites and hydrothermal veins when late-stage fluids crystallize tourmaline; specimens range from Precambrian to recent geological settings. Common sources include Brazil, Afghanistan, Pakistan, Madagascar, and the United States.

Uses & applications

Used as a collectible mineral, ornamental carving, cabochons, beads, and occasional gemstone material; tourmaline’s piezoelectric and pyroelectric properties also have technical applications.

Geological facts

Schorl is the most abundant tourmaline species and owes its black color mainly to iron. Tourmaline crystals can develop electrical charges when heated or compressed.

Field identification & locations

Identify it by its black prismatic form, lengthwise striations, glassy polish, and inability to scratch with a steel knife; confirm with hardness, density, and analytical testing because polished obsidian or industrial glass can look similar.