mineral

Polished black tourmaline (schorl)

Schorl, a tourmaline-group borosilicate: NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

AI-generated identification · may be incorrect

Hardness 7–7.5 Mohs; opaque black, vitreous to submetallic luster, commonly elongated and striated with trigonal crystals; no prominent cleavage, specific gravity about 3.0–3.3. The specimen appears tumbled or polished, with rounded edges and longitudinal grooves.

Identified September 5, 2026More mineral →

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Physical properties

Formation & geological history

Forms in boron-rich granite pegmatites and hydrothermal veins, typically during late-stage crystallization of felsic magmas; individual crystals range from Precambrian to recent geological settings. Common localities include Brazil, Afghanistan, Madagascar, Namibia, and the United States.

Uses & applications

Used as a decorative and lapidary stone, in jewelry, mineral collections, and occasionally for tourmaline’s piezoelectric and pyroelectric properties. It is not generally used as a major construction material.

Geological facts

Schorl is the most common tourmaline species and owes its black color mainly to iron. Heating or pressure can produce electrical charge, a characteristic historically used in early scientific instruments.

Field identification & locations

Identify it by its dense black color, strong longitudinal striations, rounded triangular crystal cross-section, and resistance to scratching by steel; confirm with hardness and specific-gravity tests. Approximate value: $5–$30 for a common polished piece, or $30–$100+ if unusually large or well-finished.