sedimentary
Greywacke with Quartz Veins
Lithic Sandstone (Greywacke) with SiO2 inclusions
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal matrix with white/cream veins; Luster: Dull/earthy matrix with vitreous (glassy) veins; Structure: Clastic matrix with crystalline hydrothermal veins; Cleavage: None (conchoidal fracture on quartz).
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to charcoal matrix with white/cream veins
- Luster
- Dull/earthy matrix with vitreous (glassy) veins
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal matrix with white/cream veins; Luster: Dull/earthy matrix with vitreous (glassy) veins; Structure: Clastic matrix with crystalline hydrothermal veins; Cleavage: None (conchoidal fracture on quartz).
Formation & geological history
Formed from the rapid deposition of muddy sand in deep-marine environments (turbidity currents). The white veins formed later when tectonic stress cracked the rock, allowing mineral-rich hydrothermal fluids (mostly silica) to precipitate in the fractures.
Uses & applications
Primarily used as high-quality road aggregate, railway ballast, and in heavy construction due to its extreme hardness and resistance to crushing. Occasionally used as decorative stones in landscaping.
Geological facts
Greywacke is often called 'dirty sandstone' because of its poorly sorted mixture of clay, quartz, and feldspar. The 'crack-seal' texture of the veins indicates the rock underwent multiple cycles of fluid pressure build-up and fracturing.
Field identification & locations
Identify by its dark, fine-grained, 'gritty' feel and the presence of intersecting hard white veins that stand out in relief. Common in mountainous coastal regions (e.g., California Coast Ranges, New Zealand, Wales).
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