Gneiss (Metamorphic Cobble)

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Banded grey, white, green, and pink; Luster: Variable (pearly to dull when weathered); Crystal structure: Coarse-grained phaneritic; Cleavage: None (fractures irregularly); Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). The distinct banding, or foliation, occurs under intense heat and pressure, often during mountain-building events (orogenies).

Uses & Applications

Used as crushed stone for road construction, railroad ballast, and building stone. Highly polished specimens are used as decorative flooring, countertops, or facing stones in architecture.

Geological Facts

Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The banding is caused by the segregation of light minerals (quartz/feldspar) and dark minerals (biotite/amphibole).

Field Identification & Locations

Identify by looking for 'gneissose banding'—distinct layers of different colors that are thicker than schist layers. To identify in the field, look for interlocking coarse grains and a lack of easy splitting along planes. Common in continental shields and mountain ranges.

Identified on: 4/24/2026

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