igneous
Quartz–feldspar pegmatite fragment
Granitic pegmatite, chiefly quartz (SiO₂) and feldspar, with probable biotite or chlorite
AI-generated identification · may be incorrectCoarse, interlocking pale crystals with white, cream, gray, and green-black areas; vitreous quartz and pearly feldspar luster. Hardness about 6–7 (feldspar–quartz), nonfoliated, with feldspar cleavage and quartz lacking cleavage; specific gravity roughly 2.6–2.7.
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Physical properties
Coarse, interlocking pale crystals with white, cream, gray, and green-black areas; vitreous quartz and pearly feldspar luster. Hardness about 6–7 (feldspar–quartz), nonfoliated, with feldspar cleavage and quartz lacking cleavage; specific gravity roughly 2.6–2.7.
Formation & geological history
Forms when the final, water-rich melt from a granitic magma crystallizes slowly in dikes or pockets, producing unusually large crystals. Pegmatites are generally Precambrian to Phanerozoic, depending on the host intrusion.
Uses & applications
Used as a source of feldspar, quartz, mica, and occasionally lithium or rare metals; attractive pieces are collected, while larger pegmatite is sometimes used decoratively.
Geological facts
The rounded shape appears naturally weathered or tumbled. The dark green-black material may be altered biotite, chlorite, or amphibole; exact identification requires a fresh surface or testing.
Field identification & locations
Common in granitic terrains and pegmatite districts worldwide, including Maine, California, Brazil, Madagascar, and Australia. A glassy scratch-resistant area suggests quartz; feldspar commonly shows flat cleavage faces and scratches glass less readily.
