igneous
Quartz-rich granite or granitic gneiss cobble
Granite; primarily quartz (SiO₂), feldspar, and minor mica/mafic minerals
AI-generated identification · may be incorrectLight gray, coarse crystalline, and typically subvitreous to dull; quartz has Mohs hardness 7, feldspar 6–6.5, and the rock’s specific gravity is about 2.6–2.7. The reddish patch likely represents iron-stained feldspar or a minor iron-oxide inclusion; the image does not clearly show foliation.
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Physical properties
Light gray, coarse crystalline, and typically subvitreous to dull; quartz has Mohs hardness 7, feldspar 6–6.5, and the rock’s specific gravity is about 2.6–2.7. The reddish patch likely represents iron-stained feldspar or a minor iron-oxide inclusion; the image does not clearly show foliation.
Formation & geological history
Formed by slow cooling of silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic plutonic episodes; later weathering and glacial or stream transport rounded the cobble.
Uses & applications
Widely used as crushed aggregate, building stone, paving, monuments, and countertops; this individual specimen has modest collecting value.
Geological facts
Granite commonly contains interlocking crystals visible without magnification, unlike glassy volcanic rocks. Its rounded form indicates transport rather than formation in place.
Field identification & locations
Look for visible quartz, white or pink feldspar, and scattered dark mica, with no bubbles or glassy texture; common in continental mountain belts and glacial deposits. A streak, hardness, and fresh broken surface would improve identification.
