sedimentary
Graywacke
Lithic Sandstone (Graywacke)
Hardness: 6-7 (Mohs). Color: Grey, greenish-grey, or black. Luster: Dull/Earthy. Texture: Fine-to-coarse grained with visible matrix. Cleavage: None (conchoidal fracture).
- Hardness
- 6-7 (Mohs)
- Color
- Grey, greenish-grey, or black
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs). Color: Grey, greenish-grey, or black. Luster: Dull/Earthy. Texture: Fine-to-coarse grained with visible matrix. Cleavage: None (conchoidal fracture).
Formation & geological history
Formed by rapid underwater debris flows (turbidites) in deep-sea trenches. These deposits accumulate near continental margins during tectonic activity. Most are Paleozoic to Mesozoic in age.
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and railway ballast. Often used as decorative garden stones in landscaping.
Geological facts
Graywacke is often called 'dirty sandstone' because of its poorly sorted mixture of minerals and mud. It is characteristically associated with ancient oceanic subduction zones.
Field identification & locations
Identify by its extreme hardness, dull grey appearance, and lack of visible layering compared to shale. Found commonly in mountainous beach regions and riverbeds near uplifted marine sediments.
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