igneous
Porphyritic granite pebble
Granite, composed chiefly of quartz and feldspar with dark biotite or hornblende
AI-generated identification · may be incorrectCoarse-grained, light pinkish-tan and black; interlocking crystals with no cleavage across the rock. Typical hardness is about 6–7 Mohs; the rounded shape and smooth surface indicate natural water abrasion or tumbling.
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Physical properties
Coarse-grained, light pinkish-tan and black; interlocking crystals with no cleavage across the rock. Typical hardness is about 6–7 Mohs; the rounded shape and smooth surface indicate natural water abrasion or tumbling.
Formation & geological history
Granite forms when silica-rich magma cools slowly underground, allowing visible crystals to grow. Its exact geological age cannot be determined from the photograph; such pebbles are commonly eroded from ancient continental crust.
Uses & applications
Granite is widely used as building stone, paving, countertops, and aggregate; this pebble is mainly decorative or a collecting specimen.
Geological facts
The black patches are likely biotite or hornblende, while the pale material is feldspar and quartz; the image does not show enough detail to confirm the minerals or a porphyritic texture.
Field identification & locations
Look for a hard, speckled rock with interlocking pale and dark grains; granite pebbles occur in river gravels and glacial deposits worldwide. A hand specimen like this is commonly worth about $1–$10.
