metamorphic
Gneiss (Metamorphic Pebble)
Gneiss (SiO2-rich metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Speckled gray, black, pinkish-white; Luster: Dull to sub-vitreous; Texture: Medium to coarse-grained, showing weak to moderate foliation (gneissic banding); Cleavage: None, rough to rounded fracture.
- Hardness
- 6-7 on Mohs scale
- Color
- Speckled gray, black, pinkish-white
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Speckled gray, black, pinkish-white; Luster: Dull to sub-vitreous; Texture: Medium to coarse-grained, showing weak to moderate foliation (gneissic banding); Cleavage: None, rough to rounded fracture.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks under high temperature and intense directional pressure. This specimen has been rounded into a pebble by stream or beach water erosion.
Uses & applications
Crushed stone for road construction, aggregate, landscaping pebbles, decorative garden rocks, and stone carving/dimension stone when found in larger blocks.
Geological facts
Gneiss contains the same minerals as granite (quartz, feldspar, mica, hornblende) but is distinguished by its distinct banded or speckled metamorphic texture caused by intense pressure.
Field identification & locations
Look for alternating bands or speckled clusters of light minerals (feldspar and quartz) and dark minerals (biotite mica or amphibole). Commonly found in riverbeds, glacial tills, and mountainous metamorphic terranes worldwide.
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