mineral

Polished black tourmaline (schorl)

Schorl, NaFe²⁺₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

AI-generated identification · may be incorrect

Hardness 7–7.5 Mohs; black to bluish-black, commonly vitreous to resinous luster, with strongly elongated trigonal-prismatic crystal structure and excellent longitudinal striations. The rounded form appears polished or naturally waterworn, with visible parallel tourmaline growth bands.

Identified September 9, 2026More mineral →

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Physical properties

Formation & geological history

Forms mainly in boron-rich granitic pegmatites and hydrothermal veins as magma cools; most specimens are Paleozoic to younger, though exact age depends on locality. Common sources include Brazil, Afghanistan, Pakistan, Madagascar, Namibia, and the United States.

Uses & applications

Used as a gemstone, carving material, ornamental stone, and collector specimen; gem-quality schorl is less valuable than transparent colored tourmalines.

Geological facts

Black tourmaline is strongly pleochroic and can become electrically polarized when heated or compressed (pyroelectric and piezoelectric). Its lengthwise grooves are a key diagnostic feature.

Field identification & locations

Identify by opaque black color, high hardness, longitudinal striations, and prismatic cleavage/irregular fracture; it is commonly found in pegmatite pockets with quartz and feldspar. A polished surface can obscure crystal faces, so hardness and streak tests should be cautious.