sedimentary

Graywacke

Argillaceous sandstone

Hardness: 6-7 on Mohs scale; Color: Dark gray, greenish-gray, or blackish; Luster: Dull/Earthy; Crystal structure: Clastic (sand-sized grains in a fine clay matrix); Cleavage: None, breaks with sub-conchoidal fracture.

Hardness: 6-7 on Mohs scale
Color: Dark gray, greenish-gray, or blackish
Luster: Dull/Earthy

Identified July 12, 2026More sedimentary →

Explore Graywacke in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed by rapid deposition in deep marine environments via submarine landslides or turbidity currents. These rocks are typically found in geosynclinal belts near continental margins.

Uses & applications

Primarily used as high-quality road aggregate, railway ballast, and in heavy construction where a durable, hard-wearing stone is required.

Geological facts

Graywacke is often called 'dirty sandstone' because of its poorly sorted mixture of sand grains and clay. It is a key rock for understanding tectonic movements and ancient seafloor environments.

Field identification & locations

Identify by its characteristic dark color, extreme hardness compared to other sandstones, and the presence of angular rock fragments within the darker matrix. Commonly found in mountainous coastal regions like New Zealand, Wales, and parts of the Appalachian range.