Graywacke

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark gray, greenish-gray, or brown; Luster: Dull/Earthy; Structure: Clastic, poorly sorted; Specific Gravity: 2.6-2.7. It features a matrix-rich texture with angular sand-sized grains.

Formation & Geological History

Formed in deep-ocean marine environments (submarine fans) through turbidity currents. They are primarily Paleozoic or Mesozoic in age, formed by the rapid deposition of poorly sorted sediment.

Uses & Applications

Primarily used as construction aggregate, road base, and crushed stone for railway ballast. Not typically used in jewelry due to its plain appearance.

Geological Facts

Graywacke is often called 'dirty sandstone' because of its high clay and silt matrix (more than 15%). It is a classic indicator of past tectonic activity and deep-water deposition.

Field Identification & Locations

Identify by its hardness (will scratch glass), dark color, and visible angular grains of quartz and feldspar set in a fine-grained matrix. Commonly found in mountain belts and coastal regions like the Appalachians or the New Zealand Alps.

Identified on: 5/19/2026

Mode: Standard