sedimentary
Greywacke with Quartz Veins
Lithic Wacke (SiO2 enriched)
Hardness: 6-7 (Mohs); Color: Dark grey matrix with white veins; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: None; Specific Gravity: 2.6-2.7.
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey matrix with white veins
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey matrix with white veins; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: None; Specific Gravity: 2.6-2.7.
Formation & geological history
Formed in deep-water marine environments (turbidites) during the Paleozoic to Mesozoic eras. The white veins are later hydrothermal quartz injections into fractures.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, and coastal protection (rip-rap). Small water-worn pebbles are used decoratively in landscaping.
Geological facts
Greywacke is often called 'dirty sandstone' because it contains a mix of poorly sorted grains and clay. The distinctive white 'wishing lines' are quartz filled into tectonic cracks.
Field identification & locations
Identify by its 'salt and pepper' appearance and hardness (it will scratch glass). Commonly found in mountainous fold belts and along rocky coastlines like the UK or New Zealand.
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