igneous
Quartz-rich granite or granitic gneiss cobble
Granite; primarily quartz, feldspar, and mica, possibly with gneissic deformation
AI-generated identification · may be incorrectCoarse, interlocking grains with translucent gray quartz, pale feldspar, and dark mica or amphibole; typically Mohs 6–7, vitreous-to-pearly luster, no cleavage as a whole, and specific gravity about 2.6–2.8. The rounded shape reflects stream or beach abrasion rather than its original texture.
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Physical properties
Coarse, interlocking grains with translucent gray quartz, pale feldspar, and dark mica or amphibole; typically Mohs 6–7, vitreous-to-pearly luster, no cleavage as a whole, and specific gravity about 2.6–2.8. The rounded shape reflects stream or beach abrasion rather than its original texture.
Formation & geological history
Formed by slow crystallization of silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic mountain-building; if foliated, it may have been metamorphosed into gneiss. Erosion transported and rounded this specimen in a sedimentary environment.
Uses & applications
Granite is widely used for building stone, countertops, monuments, aggregate, and landscaping; attractive pieces may be collected but this rounded cobble has little commercial value.
Geological facts
Quartz appears glassy and gray, feldspar is commonly white or pink, and mica forms dark reflective flakes. A weak banding or aligned mineral texture would favor granitic gneiss over ordinary granite.
Field identification & locations
Common in continental crust and stream gravels worldwide, especially regions underlain by granitic batholiths or ancient shields. A hand lens, streak observation, and acid test can help; lack of fizz distinguishes it from limestone.
