metamorphic

Gneiss (rounded cobble)

Quartz–feldspar–mica gneiss; exact mineral proportions cannot be determined from the image

AI-generated identification · may be incorrect

Coarse-grained, gray-speckled rock with faint streaking or foliation and a dull to sparkling luster. Typical minerals are quartz, feldspar, and biotite or other mica; hardness is roughly 6–7 Mohs, with no consistent cleavage across the rock.

Identified October 4, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when pre-existing igneous or sedimentary rock was recrystallized under high heat and pressure, commonly deep in continental crust. Gneiss occurs across a wide range of geological ages; this specimen’s age and source cannot be identified visually.

Uses & applications

Used as decorative and dimension stone, paving, landscaping aggregate, and building material; rounded cobbles are often collected or used in gardens.

Geological facts

Gneiss commonly shows mineral segregation into light and dark bands, though some specimens have weak or indistinct banding. Its rounded shape likely reflects transport and abrasion in water or glacial deposits.

Field identification & locations

Look for coarse interlocking grains and streaks or bands of contrasting minerals; distinguish it from granite by foliation or mineral alignment. Gneiss is common in ancient continental terrains and glacial or river gravels.