igneous
Pegmatitic quartz–feldspar rock with dark mica or garnet
Granitic pegmatite; chiefly quartz (SiO₂), feldspar, and biotite/muscovite, possibly garnet
AI-generated identification · may be incorrectCoarse interlocking crystals, white to pink translucent quartz/feldspar, and dark flaky or granular minerals; vitreous quartz luster and uneven fracture. Quartz hardness is 7, feldspar 6–6.5, and mica about 2–3; cleavage is prominent in feldspar and mica.
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Physical properties
Coarse interlocking crystals, white to pink translucent quartz/feldspar, and dark flaky or granular minerals; vitreous quartz luster and uneven fracture. Quartz hardness is 7, feldspar 6–6.5, and mica about 2–3; cleavage is prominent in feldspar and mica.
Formation & geological history
Formed from the final, water-rich fluids of a slowly cooling granitic magma, producing exceptionally large crystals. Most pegmatites formed during Precambrian to Phanerozoic mountain-building events.
Uses & applications
Used locally as decorative stone and a source of quartz, feldspar, mica, lithium, and rare minerals; attractive specimens are collected, though this piece is not clearly gem-grade.
Geological facts
Pegmatites commonly host tourmaline, beryl, spodumene, and rare-element minerals. The blocky pale crystals and very coarse texture distinguish pegmatite from ordinary granite.
Field identification & locations
Identify by its extreme crystal size, interlocking quartz/feldspar, and mica-rich dark zones; common in granitic mountain belts worldwide, including Brazil, Madagascar, Maine, and California. A hand lens and streak/cleavage observations help confirm minerals.
