igneous
Tourmaline-bearing pegmatite, likely schorl
Black tourmaline (schorl), NaFe²⁺₃Al₆(BO₃)₃Si₆O₁₈(OH)₄, in quartz-rich pegmatite
AI-generated identification · may be incorrectThe specimen shows elongated, strongly striated prismatic black-to-dark green crystals with vitreous luster, embedded in tan quartz/feldspar matrix. Tourmaline has Mohs hardness 7–7.5, poor cleavage, trigonal structure, and specific gravity about 3.0–3.3; the greenish color may reflect dravite or iron-rich schorl.
Identified More igneous →
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Physical properties
The specimen shows elongated, strongly striated prismatic black-to-dark green crystals with vitreous luster, embedded in tan quartz/feldspar matrix. Tourmaline has Mohs hardness 7–7.5, poor cleavage, trigonal structure, and specific gravity about 3.0–3.3; the greenish color may reflect dravite or iron-rich schorl.
Formation & geological history
Formed from volatile-rich granitic pegmatite fluids during late-stage cooling, commonly in Precambrian to Phanerozoic granite bodies. Exact age and species require locality data or chemical testing.
Uses & applications
Used as a collector mineral; transparent colored tourmaline is cut as a gemstone, while opaque schorl is mainly ornamental. Pegmatite quartz and feldspar may have industrial value, but this hand specimen is primarily collectible.
Geological facts
Tourmaline crystals are commonly longitudinally striated and can become electrically polarized when heated or rubbed (the origin of “tourmaline” detectors).
Field identification & locations
Identify by prismatic striated crystals, high hardness, lack of cleavage, and quartz/feldspar matrix; common localities include Brazil, Afghanistan, Pakistan, Madagascar, Namibia, and the United States. Confirm with hardness, streak, and ideally Raman or chemical analysis.
