sedimentary

Greywacke with Quartz Veins

Lithic Sandstone (Greywacke) with SiO2 inclusions

AI-generated identification · may be incorrect

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

Identified March 6, 2026More sedimentary →

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Physical properties

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).