igneous
Rounded quartz-rich cobble, likely granite or granitic gneiss
Granite/granitic gneiss; mainly quartz, feldspar, and mica
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, coarse-grained, granular texture with silvery mica and white feldspar, vitreous-to-dull luster, and no obvious cleavage as a whole rock. Its rounded shape reflects abrasion during transport rather than natural crystal form.
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Physical properties
Hardness about 6–7 Mohs; gray, coarse-grained, granular texture with silvery mica and white feldspar, vitreous-to-dull luster, and no obvious cleavage as a whole rock. Its rounded shape reflects abrasion during transport rather than natural crystal form.
Formation & geological history
Likely formed deep in the continental crust from slowly cooled magma, or from metamorphosed granite if the faint banding is genuine; typical ages range from hundreds of millions to billions of years. It was later weathered and transported by water or glacial activity.
Uses & applications
Used as aggregate, landscaping stone, riprap, and building stone; attractive polished pieces may be sold as decorative specimens. This ordinary cobble has little commercial gemstone value.
Geological facts
The specimen appears to contain abundant interlocking quartz and feldspar grains, with darker mica-rich patches. Surface rounding and weathering can obscure the original rock texture.
Field identification & locations
Look for visible quartz (translucent gray), blocky white feldspar, and black or silvery mica; a steel scratch test should not easily scratch quartz. Common in glacial deposits, streambeds, beaches, and areas underlain by granite or gneiss.
