metamorphic

Gneiss (likely garnet-bearing)

Banded orthogneiss/paragneiss; chiefly quartz, feldspar, biotite and amphibole, with possible garnet

AI-generated identification · may be incorrect

Hardness about 6–7 Mohs; coarse, interlocking crystals with gray, white, black and rusty-brown patches, vitreous to dull luster, and weakly visible compositional banding. Rounded edges indicate a water-worn cobble or tumbled specimen.

Identified September 14, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or sedimentary protolith was subjected to intense heat and pressure during regional metamorphism, commonly at roughly 500–800°C. Such rocks are generally Precambrian to Paleozoic in age, though the specimen’s age cannot be determined visually.

Uses & applications

Used as crushed aggregate, dimension stone, landscaping rock and decorative slabs; attractive pieces are collected but are not typically gem material.

Geological facts

The rusty coloration usually reflects oxidized iron in biotite, amphibole or garnet. Gneiss commonly preserves alternating light quartz-feldspar and dark mica-rich layers produced by metamorphic segregation.

Field identification & locations

Identify it by coarse grains, mixed light/dark mineral bands, substantial hardness, and lack of consistent cleavage; a streak test is usually white to gray. Common in continental shield regions and glacial or river deposits worldwide.