igneous
Rounded granite or granitic gneiss cobble
Granite; predominantly quartz, feldspar, and mica (approximate identification)
AI-generated identification · may be incorrectDark gray, speckled, fine- to medium-grained, with a dull to weakly glassy luster and no obvious cleavage. Typical granite hardness is about Mohs 6–7 and specific gravity 2.6–2.7; its smooth shape reflects abrasion rather than crystal structure.
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Physical properties
Dark gray, speckled, fine- to medium-grained, with a dull to weakly glassy luster and no obvious cleavage. Typical granite hardness is about Mohs 6–7 and specific gravity 2.6–2.7; its smooth shape reflects abrasion rather than crystal structure.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep underground, then weathered and transported by water or glacial action; the exact geological age cannot be determined from the image. If foliated internally, it may instead be granitic gneiss, a metamorphic equivalent.
Uses & applications
Used extensively as crushed aggregate, building stone, paving, monuments, and countertops; this rounded specimen has little commercial value but may be a useful teaching or display sample.
Geological facts
The visible light and dark grains are typical of interlocking quartz–feldspar–mica textures. Rounded cobbles commonly form in river channels, beaches, and glacial deposits.
Field identification & locations
Look for visible interlocking grains, abundant pale feldspar/quartz, dark mica or amphibole, high hardness, and resistance to scratching; vinegar should not fizz. Common in continental mountain belts and river gravels worldwide.
