metamorphic
Gneissic River Cobble
Gneiss
Hardness: 6-7 (Mohs scale); Color: Dark grey with faint banding; Luster: Dull to slightly silky; Crystal structure: Coarse-grained foliated; Cleavage: Poor, splits along foliation planes; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey with faint banding
- Luster
- Dull to slightly silky
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey with faint banding; Luster: Dull to slightly silky; Crystal structure: Coarse-grained foliated; Cleavage: Poor, splits along foliation planes; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under high temperature and pressure. This specimen has been rounded and smoothed by water transport in a river or glacial environment.
Uses & applications
Mainly used as decorative landscaping stone, crushed stone for construction, or as river rocks in architectural design. High-quality gneiss can be used as dimension stone for countertops.
Geological facts
Gneiss is some of the oldest rock found on Earth, with some specimens in Canada dating back 4 billion years. The banding occurs due to the segregation of different mineral types during intense pressure.
Field identification & locations
Identify in the field by looking for distinct bands of light and dark minerals (micas, quartz, feldspar). Common in mountain ranges and glacial till. This specific piece is a well-rounded 'river rock'.
