metamorphic
Gneiss (Metamorphic Rock Pebble)
Foliated Metamorphic Rock (Quartz-Feldspar Gneiss / Granitic Gneiss)
Hardness: 6-7 on Mohs scale. Color: Mottled gray, greenish-gray, whitish-pink, and brownish tones. Luster: Dull to vitreous on fresh breaks. Texture: Medium-to-coarse grained, water-worn and rounded pebble with visible crystalline mineral grains including quartz, feldspar, and ferromagnesian minerals.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous on fresh breaks
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Mottled gray, greenish-gray, whitish-pink, and brownish tones. Luster: Dull to vitreous on fresh breaks. Texture: Medium-to-coarse grained, water-worn and rounded pebble with visible crystalline mineral grains including quartz, feldspar, and ferromagnesian minerals.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks under high temperature and extreme pressure deep within the Earth's crust. Later eroded, transported, and rounded by river or glacial action.
Uses & applications
Crushed stone for road construction, ballast, and aggregate in concrete; decorative landscaping pebbles; educational specimens.
Geological facts
Gneiss is one of the oldest rocks found on Earth, with some specimens like the Acasta Gneiss in Canada dating back over 4 billion years. Its characteristic banding results from mineral segregation during severe thermal and pressure stress.
Field identification & locations
Identify by its interlocking crystalline texture, granular composition of quartz and feldspar, subtle banding or alignment of grains, and high hardness. Commonly found along riverbeds, beaches, and glacial till deposits globally.
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