sedimentary
Greywacke (River Pebble)
Greywacke sandstone with quartz veining
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal with thin white quartz veins; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained matrix); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to charcoal with thin white quartz veins
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal with thin white quartz veins; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained matrix); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the rapid deposition of muddy sand in deep ocean environments (turbidites). This specimen has been rounded by fluvial (river) erosion and likely dates back to the Paleozoic or Mesozoic eras depending on local regional geology.
Uses & applications
Primarily used as high-quality road aggregate, railway ballast, and in concrete production due to its extreme hardness and durability. Small pebbles are often used in decorative landscaping and aquariums.
Geological facts
Greywacke is often called 'dirty sandstone' because it contains a large amount of clay and silt mixed with sand grains. The white lines visible are 'veins' created when silica-rich hydrothermal fluids filled cracks in the rock and crystallized into quartz.
Field identification & locations
Identify in the field by its dark, uniform grey color, extreme toughness (it doesn't break easily with a hammer), and the presence of fine quartz web-like veins. Commonly found in riverbeds near mountainous tectonic regions like New Zealand, the UK, and the Appalachian or Pacific ranges of North America.
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