Greywacke

A hard, dark 'dirty' sandstone of mixed grains, laid down by underwater avalanches.

Greywacke specimen — Photo: James St. John, CC BY 2.0, via Wikimedia Commons

How to identify greywacke

Dark gray and very hard.
Mix of sand grains and rock fragments in fine matrix.
Breaks into sharp, angular blocks.

Often confused with

Sandstone
Clean sandstone is lighter and grains are more uniform.
Compare side by side
Basalt
Basalt has no sand grains.
Compare side by side

Where it's found

Southern Alps, New Zealand
Wales
Harz Mountains, Germany

Uses

Road aggregate.

Rock tumbling

Can be tumbled with care.

Did you know?

The name comes from German Grauwacke, an old miners' term.

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