sedimentary

Greywacke (River Pebble with Quartz Veins)

Greywacke (Indurated Sandstone); Veins: SiO2 (Quartz)

Hardness: 6-7 on Mohs scale; Color: Dull grey to greenish-grey matrix with white veins; Luster: Earthy basis with vitreous veins; Crystal Structure: Clastic (matrix) and Trigonal (veins); Specific Gravity: 2.6-2.7

Hardness: 6-7 on Mohs scale
Color: Dull grey to greenish-grey matrix with white veins
Luster: Earthy basis with vitreous veins

Identified March 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the rapid deposition of muddy sand in deep-sea environments (turbidity currents), then subjected to pressure. The white veins formed later when mineral-rich hydrothermal fluids filled fractures. Usually Paleozoic to Mesozoic in age.

Uses & applications

Commonly used in construction as road aggregate, railroad ballast, or decorative landscaping stones. Smooth river-worn specimens are popular for rock painting and garden features.

Geological facts

Greywacke is often called 'dirty sandstone' because it contains a high percentage of clay and rock fragments. The white lines are tectonic signatures, showing where the rock cracked under stress and was 'healed' by quartz.

Field identification & locations

Identify by its fine-grained grey texture, extreme hardness (won't scratch with a copper penny), and branching white quartz veins. Commonly found in riverbeds, glacial tills, and coastal beaches globally.

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