metamorphic

Banded gneiss pebble

Gneiss, commonly quartz–feldspar–mica banded rock

AI-generated identification · may be incorrect

Medium-grained, gray to brown-gray, with distinct light and dark compositional bands and a dull to slightly silky luster. Typical hardness is about 6–7 Mohs; grains are interlocking, with no obvious cleavage and a specific gravity near 2.6–2.8.

Identified September 15, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting granite or sedimentary rock underwent intense heat and pressure, causing minerals to recrystallize and segregate into bands. Most gneiss is Precambrian to Paleozoic, though its exact age cannot be determined from the photograph.

Uses & applications

Used as crushed aggregate, road stone, building stone, countertops, and landscaping material; attractive polished pieces are collected but generally have little gem value.

Geological facts

The banding is called gneissic foliation and records deformation deep within Earth’s crust. The rounded shape indicates transport and abrasion in a river, beach, or glacial setting.

Field identification & locations

Identify it by alternating pale quartz/feldspar-rich and dark biotite/amphibole-rich layers, unlike uniformly speckled granite. Common in continental shields and metamorphic mountain belts; a hand lens and fresh broken surface improve identification.