igneous
Quartz-rich pegmatite (possibly tourmaline-bearing)
Granitic pegmatite; chiefly quartz (SiO₂), feldspar, and dark mica/amphibole
AI-generated identification · may be incorrectCoarse interlocking crystals with white to translucent quartz/feldspar and black mica or tourmaline; vitreous quartz luster, uneven fracture, Mohs hardness about 6–7, and specific gravity roughly 2.6–2.8. The apparent banding is mineral zoning rather than sedimentary layering.
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Physical properties
Coarse interlocking crystals with white to translucent quartz/feldspar and black mica or tourmaline; vitreous quartz luster, uneven fracture, Mohs hardness about 6–7, and specific gravity roughly 2.6–2.8. The apparent banding is mineral zoning rather than sedimentary layering.
Formation & geological history
Forms when volatile-rich granitic magma crystallizes slowly in dikes and pockets, producing exceptionally large crystals. Most pegmatites are Precambrian to Cenozoic, depending on locality.
Uses & applications
Used as a source of feldspar, quartz, mica, lithium, and rare-element minerals; attractive pieces are collected as decorative specimens. This rough fragment has limited jewelry value unless gem-quality crystals are present.
Geological facts
Pegmatites can host tourmaline, beryl, spodumene, garnet, and lithium minerals. Dark areas may be biotite mica, hornblende, or black tourmaline; a photograph cannot distinguish them confidently.
Field identification & locations
Look for very coarse crystals, glassy quartz, blocky feldspar, and flaky mica; it is commonly found in granitic terrains worldwide, including Brazil, Madagascar, Maine, and California. A streak test, hardness test, and close inspection of crystal faces would improve identification.
