igneous
Quartz-rich pegmatitic rock (likely milky quartz with feldspar and dark mica)
Pegmatite; predominantly quartz (SiO₂), with feldspar and biotite/hornblende
AI-generated identification · may be incorrectCoarse, uneven white-to-gray mass with vitreous to dull luster and dark inclusions; quartz is Mohs 7, feldspar 6, and mica about 2–3. Crystals are intergrown and lack obvious foliation; exact composition requires testing.
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Physical properties
Coarse, uneven white-to-gray mass with vitreous to dull luster and dark inclusions; quartz is Mohs 7, feldspar 6, and mica about 2–3. Crystals are intergrown and lack obvious foliation; exact composition requires testing.
Formation & geological history
Formed when water-rich granitic magma cooled slowly, allowing exceptionally large crystals to grow, typically during Precambrian to Phanerozoic plutonic events. Common in veins and dikes cutting older granitic or metamorphic rocks.
Uses & applications
Used as a source of quartz and feldspar for glass, ceramics, and engineered stone; attractive pieces may be sold as display specimens, though this example appears fairly ordinary.
Geological facts
Pegmatites can host spectacular crystals of tourmaline, beryl, spodumene, and rare-element minerals. The dark material may be biotite mica or amphibole, while the white material is likely milky quartz and feldspar.
Field identification & locations
Look for very coarse crystals, glassy quartz that scratches glass, feldspar cleavage, and flaky dark mica. Common worldwide in granitic terrains, including Maine, California, Brazil, Madagascar, and Scandinavia.
