metamorphic

Banded gneiss cobble

Gneiss, typically quartz–feldspar–mica assemblage

AI-generated identification · may be incorrect

Medium- to coarse-grained, with alternating dark mafic and light quartz-feldspar bands; commonly vitreous to dull luster, Mohs hardness about 6–7, non-foliated cleavage at the grain scale but strong compositional banding, and specific gravity roughly 2.6–2.8.

Identified September 5, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or other preexisting rock underwent high-grade regional metamorphism, recrystallizing and segregating minerals into bands during intense heat and pressure; individual specimens may be Precambrian to much younger. The rounded shape indicates stream or glacial transport.

Uses & applications

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

Geological facts

The light bands are quartz and feldspar, while darker bands commonly contain biotite, amphibole, or pyroxene. It may resemble banded granite, but gneiss shows more pronounced mineral segregation and foliation.

Field identification & locations

Identify it by coarse interlocking grains and irregular light-dark banding that persists across the cobble; common in old continental shield regions and metamorphic mountain belts. A photo cannot confirm exact mineralogy without testing.