metamorphic

Weathered granitic gneiss (probable)

Gneiss, chiefly quartz–feldspar–biotite; compositionally granitic

AI-generated identification · may be incorrect

Gray, coarse-grained, weakly foliated rock with visible quartz and feldspar and dark biotite/hornblende; quartz hardness 7, feldspar 6, biotite 2.5–3, typically nonmetallic luster and poor-to-absent cleavage as a whole.

Identified September 4, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granitic or other felsic protolith was deeply buried and metamorphosed under high temperature and pressure, commonly during Precambrian to Paleozoic mountain building; surface weathering has rounded the specimen.

Uses & applications

Used as crushed aggregate, riprap, building stone, and occasionally dimension stone; attractive foliated pieces are collected but are generally low-value.

Geological facts

The image shows a rounded cobble amid mixed sediment, so exact identification is limited; it may also be strongly weathered granite rather than true gneiss.

Field identification & locations

Look for alternating light quartz–feldspar and dark mineral bands or streaks, coarse interlocking grains, and a very hard texture that scratches glass. Common in continental shields, glacial deposits, river gravels, and mountain belts.