mineral
Polished black tourmaline (schorl)
Schorl, a sodium–iron tourmaline; approximate formula NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
AI-generated identification · may be incorrectBlack, vitreous to submetallic luster, commonly striated; Mohs hardness 7–7.5. Tourmaline has a trigonal crystal structure and typically lacks obvious cleavage; this appears to be a polished specimen, so its crystal form is obscured.
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Physical properties
Black, vitreous to submetallic luster, commonly striated; Mohs hardness 7–7.5. Tourmaline has a trigonal crystal structure and typically lacks obvious cleavage; this appears to be a polished specimen, so its crystal form is obscured.
Formation & geological history
Forms in boron-rich granitic pegmatites and some metamorphic rocks as magma cools and fluids crystallize. Tourmaline occurs in rocks of many ages; the specimen’s age cannot be determined from the image.
Uses & applications
Collected and displayed as a mineral specimen; tourmaline is also used as a gemstone, especially when transparent and well colored.
Geological facts
Schorl is the most common tourmaline species and accounts for most black tourmaline. The white patches may be an associated mineral or surface residue, not diagnostic from the image.
Field identification & locations
Check for a glassy sheen and longitudinal striations on any exposed crystal surfaces; schorl commonly occurs with quartz in pegmatites worldwide, including Brazil, Afghanistan, and the United States.
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Other mineral specimens
Gneiss or Schist
Gneiss or Mica Schist with Garnet
Metamorphic
Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
sedimentary
Tumbled granite
Granite, a coarse-grained felsic intrusive igneous rock
igneous
Pegmatite with Tourmaline and Quartz
Igneous Pegmatite
igneous
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
metamorphic
Gypsum (variety Selenite or Alabaster)
Calcium sulfate dihydrate (CaSO4·2H2O)
Mineral