metamorphic
Gneiss (Stream-worn Cobble)
Gneiss (Metamorphic rock composed primarily of Quartz, Feldspar, and Mica)
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and tan; Luster: Dull to pearly/vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Specific Gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, and tan
- Luster
- Dull to pearly/vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and tan; Luster: Dull to pearly/vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Specific Gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme heat and pressure. This specific specimen has been further shaped by fluvial erosion (water wear in a stream or river).
Uses & applications
Used as crushed stone for road construction, railroad ballast, and building stone. Highly polished slabs are used for countertops or ornamental grave markers.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The distinct banding occurs because minerals segregate into layers during intense metamorphic folding.
Field identification & locations
Identify in the field by looking for 'stripes' or layers of light and dark minerals that are not easily flaked (unlike schist). This river-tumbled specimen is smooth and rounded. Found worldwide in the roots of mountain ranges.
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