igneous
Quartz-rich pegmatite
Granitic pegmatite; predominantly quartz (SiO₂), with feldspar and dark accessory minerals such as biotite or tourmaline
AI-generated identification · may be incorrectCoarse, interlocking translucent to milky quartz crystals with white/tan feldspar and dark inclusions; vitreous luster, no obvious cleavage in quartz, Mohs hardness ~7, specific gravity ~2.6–2.7. Crystal sizes and irregular cavities are characteristic of pegmatitic texture.
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Physical properties
Coarse, interlocking translucent to milky quartz crystals with white/tan feldspar and dark inclusions; vitreous luster, no obvious cleavage in quartz, Mohs hardness ~7, specific gravity ~2.6–2.7. Crystal sizes and irregular cavities are characteristic of pegmatitic texture.
Formation & geological history
Formed when water-rich granitic magma cooled very slowly, allowing exceptionally large crystals to grow in late-stage veins or pockets. Most specimens are Phanerozoic to Precambrian, depending on the host granite and locality.
Uses & applications
Used mainly as a collector specimen; quartz and feldspar from pegmatites may be used in glass, ceramics, abrasives, and engineered stone. Attractive transparent crystals can have minor decorative or lapidary value.
Geological facts
The dark grains may be schorl tourmaline, biotite, or another accessory mineral; precise identification requires a close examination or chemical testing. Pegmatites are important sources of lithium, beryllium, tantalum, and other rare elements.
Field identification & locations
Identify by very coarse grain size, abundant quartz, and intergrown feldspar rather than uniform granite texture; a glassy quartz grain scratches glass, while feldspar shows cleavage. Common in granitic terrains worldwide, especially Brazil, Madagascar, the United States, and Norway.
