sedimentary
Greywacke (River Pebble)
Greywacke (Indurated Sandstone)
Hardness: 6-7 on Mohs scale. Color: Dull grey, greenish-grey, or brownish. Luster: Dull/Earthy. Structure: Clastic, fine to medium-grained. Cleavage: None (conchoidal fracture when broken). Specific Gravity: 2.6-2.7.
- Hardness
- 6-7 on Mohs scale
- Color
- Dull grey, greenish-grey, or brownish
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dull grey, greenish-grey, or brownish. Luster: Dull/Earthy. Structure: Clastic, fine to medium-grained. Cleavage: None (conchoidal fracture when broken). Specific Gravity: 2.6-2.7.
Formation & geological history
Formed from the lithification of poorly sorted sediment in deep-marine environments (turbidity currents). Often Paleozoic or Mesozoic in age, found in fold mountain belts.
Uses & applications
Commonly used as crushed stone for road base, railway ballast, and construction aggregate. Polished river pebbles are used in landscaping or Zen gardens.
Geological facts
Greywacke is often called 'dirty sandstone' because of its mixture of sand-sized grains within a fine clay matrix. It is a major component of the bedrock in places like New Zealand and the UK.
Field identification & locations
Identify in the field by its extreme hardness compared to slate, its grey-green hue, and its presence in riverbeds as smooth, rounded 'potatoes'. Common in coastal and mountainous regions.
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