igneous
Quartz-rich pegmatite (likely feldspar–quartz rock)
Granitic pegmatite; chiefly quartz (SiO₂) and feldspar, with minor mica and dark accessory minerals
AI-generated identification · may be incorrectCoarse, irregular interlocking crystals; cream-white feldspar, translucent gray quartz, and black mica/amphibole. Hardness is approximately 6–7 (quartz 7, feldspar 6), with vitreous quartz luster and feldspar cleavage; density commonly 2.6–2.7 g/cm³.
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Physical properties
Coarse, irregular interlocking crystals; cream-white feldspar, translucent gray quartz, and black mica/amphibole. Hardness is approximately 6–7 (quartz 7, feldspar 6), with vitreous quartz luster and feldspar cleavage; density commonly 2.6–2.7 g/cm³.
Formation & geological history
Formed when volatile-rich granitic magma cooled slowly in dikes or pockets, allowing exceptionally large crystals to grow. Pegmatites are generally late Precambrian to Phanerozoic, depending on the host intrusion.
Uses & applications
Used decoratively as rough or polished stone and collected for large crystals; some pegmatites are mined for quartz, feldspar, mica, lithium, beryllium, and rare-element minerals.
Geological facts
The specimen’s weathered surface and coarse texture strongly suggest pegmatite, though exact identification requires a fresh break or testing. Black veins may represent biotite, amphibole, or iron/manganese staining.
Field identification & locations
Look for very coarse crystals, glassy quartz, blocky feldspar cleavage, and mica flakes; common in granitic mountain belts worldwide. A hardness or streak test can distinguish quartz and feldspar.
