igneous
Rounded granite cobble
Granite; primarily quartz, alkali feldspar, plagioclase, and minor biotite/hornblende
AI-generated identification · may be incorrectCoarse-grained, light gray to tan, granular texture with visible interlocking crystals; typically Mohs hardness 6–7, vitreous quartz luster, no distinct cleavage as a whole, and specific gravity about 2.6–2.7. Its smooth oval shape results from abrasion, not crystal form.
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Physical properties
Coarse-grained, light gray to tan, granular texture with visible interlocking crystals; typically Mohs hardness 6–7, vitreous quartz luster, no distinct cleavage as a whole, and specific gravity about 2.6–2.7. Its smooth oval shape results from abrasion, not crystal form.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep in continental crust, commonly during Precambrian to Phanerozoic intrusive events. This specimen was later weathered and rounded by stream or glacial transport.
Uses & applications
Used widely as building stone, countertops, monuments, paving, aggregate, and landscaping material; ordinary cobbles have little jewelry value.
Geological facts
The white to gray grains are feldspar and quartz, while darker specks are mafic minerals. The rounded surface and granular texture distinguish it from basalt, which is usually dark and fine-grained.
Field identification & locations
Common in continental mountain belts, glacial deposits, riverbeds, and gravel bars worldwide. A hand lens reveals interlocking quartz and feldspar grains; vinegar produces no reaction, helping distinguish it from limestone.
