mineral

Polished black tourmaline (schorl)

Schorl, NaFe²⁺₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

AI-generated identification · may be incorrect

Very dark gray-black, vitreous to resinous luster, commonly striated and elongated; Mohs hardness 7–7.5, trigonal crystal system, poor cleavage, and specific gravity about 3.1–3.3. The reddish-brown areas may be iron-rich alteration, matrix, or surface reflections, so visual identification is provisional.

Identified September 10, 2026More mineral →

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Physical properties

Formation & geological history

Forms mainly in boron-rich granitic pegmatites and hydrothermal veins during late-stage crystallization, commonly in Precambrian to Phanerozoic terrains. This specimen has been tumbled or polished, obscuring crystal faces.

Uses & applications

Used as a decorative stone, jewelry material, and collector specimen; tourmaline is also studied for its piezoelectric and pyroelectric properties. Common polished pieces generally have modest commercial value.

Geological facts

Schorl is the most abundant tourmaline species and is strongly pleochroic; heating or pressure can produce electrical charges at its ends. Its longitudinal striations are a useful diagnostic feature.

Field identification & locations

Look for black color, elongated prismatic form, longitudinal striations, high hardness, and inability to scratch with a steel knife; a streak and specific-gravity or analytical test can confirm it. Common localities include Brazil, Afghanistan, Pakistan, Madagascar, Namibia, and the United States.