sedimentary

Graywacke

Lithic Sandstone (Graywacke)

Hardness: 6-7 on Mohs scale; Color: Dark gray to greenish-gray; Luster: Dull/Earthy; Crystal structure: Clastic (fine to medium grain); Specific Gravity: 2.6-2.7

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

Identified July 7, 2026More sedimentary →

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Physical properties

Hardness: 6-7 on Mohs scale; Color: Dark gray to greenish-gray; Luster: Dull/Earthy; Crystal structure: Clastic (fine to medium grain); Specific Gravity: 2.6-2.7

Formation & geological history

Formed from the rapid deposition of muddy sediment in deep marine environments, often via turbidity currents. These rocks are typically paleozoic or younger and represent poorly sorted sediment accumulation.

Uses & applications

Primarily used as high-quality aggregate for road construction, concrete production, and railroad ballast due to its extreme hardness and durability.

Geological facts

Graywacke is often called 'dirty sandstone' because of its poorly sorted mixture of clay, quartz, and feldspar. It is a signature rock of the Franciscan Complex in California.

Field identification & locations

Identify by its characteristic 'salt and pepper' appearance under a lens, its extreme toughness compared to other sandstones, and its lack of visible bedding in hand specimens. Commonly found in mountain belts and coastal ranges.