sedimentary

Greywacke

Lithic Sandstone (Greywacke)

Hardness: 6-7 (Mohs scale); Color: Dark grey, greenish-grey, or brownish; Luster: Dull to earthy; Crystal structure: Clastic (fine to medium grain mixed with silt/clay); Specific gravity: 2.6-2.7.

Hardness: 6-7 (Mohs scale)
Color: Dark grey, greenish-grey, or brownish
Luster: Dull to earthy

Identified June 15, 2026More sedimentary →

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Physical properties

Hardness: 6-7 (Mohs scale); Color: Dark grey, greenish-grey, or brownish; Luster: Dull to earthy; Crystal structure: Clastic (fine to medium grain mixed with silt/clay); Specific gravity: 2.6-2.7.

Formation & geological history

Formed from the rapid deposition of muddy sediment in deep marine environments near continental margins or volcanic island arcs. It consists of poorly sorted, angular grains accumulated through underwater landslides (turbidity currents).

Uses & applications

Primarily used as a durable aggregate in road construction, sub-base material, railway ballast, and occasionally for rough stone walls or armor stone for sea defenses.

Geological facts

Greywacke is often called 'dirty sandstone' because of its high clay and silt matrix. It is a key indicator of tectonic activity and ancient trench environments in the geological record.

Field identification & locations

Identify by its dark, 'salt and pepper' appearance and extreme hardness compared to most sandstones. Commonly found in folded mountain belts and coastal regions. It will scratch steel and does not fizz with acid.