metamorphic
Black tourmaline-bearing pegmatite or schorl-rich metamorphic rock
Schorl tourmaline, NaFe3Al6(BO3)3Si6O18(OH)4, with quartz/feldspar and iron-oxide matrix
AI-generated identification · may be incorrectDark black, uneven to subvitreous luster, opaque, and apparently massive or fractured rather than well-crystallized; tourmaline has Mohs hardness 7–7.5, no cleavage, and specific gravity about 3.0–3.3. The tan-orange patches are likely weathered feldspar, quartz, or iron oxides.
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Physical properties
Dark black, uneven to subvitreous luster, opaque, and apparently massive or fractured rather than well-crystallized; tourmaline has Mohs hardness 7–7.5, no cleavage, and specific gravity about 3.0–3.3. The tan-orange patches are likely weathered feldspar, quartz, or iron oxides.
Formation & geological history
Likely formed in a boron-rich hydrothermal or pegmatitic environment during late-stage crystallization of granitic magma, or by metamorphism of such material; its precise age cannot be determined from the image. Common associations include quartz, feldspar, mica, and iron oxides.
Uses & applications
Black tourmaline is used as a collector mineral, ornamental stone, and occasionally in jewelry; the pictured rough matrix specimen has modest commercial value.
Geological facts
Schorl is the most common tourmaline species and is strongly pleochroic in crystals; tourmaline can become electrically polarized when heated or pressured (pyroelectric and piezoelectric).
Field identification & locations
Look for prismatic, often longitudinally striated black crystals, vitreous luster, hardness that scratches glass, and strong resistance to cleavage; a magnet may show weak attraction if iron-rich. Common sources include Brazil, Afghanistan, Africa, Maine and California.
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