mineral

Tumbled black stone, likely black tourmaline

Schorl tourmaline, NaFe²⁺₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

AI-generated identification · may be incorrect

Dark black to brown-black, vitreous to resinous luster, commonly striated and elongated; Mohs hardness 7–7.5, no prominent cleavage, trigonal crystal structure, specific gravity about 3.1–3.3. The rounded surface indicates tumbling or polishing.

Identified September 10, 2026More mineral →

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Physical properties

Formation & geological history

Forms mainly in granite-related pegmatites and metamorphic hydrothermal veins as magma cools and boron-rich fluids crystallize; specimens range from Precambrian to younger Cenozoic deposits. The image alone cannot establish its exact species or age.

Uses & applications

Used as a collector mineral and gemstone, often marketed for ornamental carvings and cabochons; tourmaline is also valued for piezoelectric and pyroelectric properties. Common tumbled pieces generally have modest commercial value.

Geological facts

Black tourmaline is the most common tourmaline variety. Its strong longitudinal striations and ability to become electrically charged when heated or rubbed are diagnostic features.

Field identification & locations

Common sources include Brazil, Afghanistan, Pakistan, Madagascar, Namibia, and the United States, especially Maine and California. Confirm with hardness, striations, density, and a gemological test; iron-rich amphibole or slag can look similar.