sedimentary

Graywacke Pebble

Graywacke (immature sandstone)

Hardness: 6-7 (Mohs scale); Color: Dark gray to greenish-gray; Luster: Dull/Earthy; Crystal structure: Clastic (fine to medium grained matrix with small fragments); Cleavage: None (conchoidal to uneven fracture); Specific Gravity: 2.6-2.7

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

Identified June 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed via rapid deposition of muddy sediment in deep marine environments, typically through submarine landslides or turbidity currents. Often Paleozoic or Mesozoic in age.

Uses & applications

Used primarily as crushed stone for road construction, railroad ballast, and high-strength concrete aggregate because of its extreme hardness and durability.

Geological facts

Graywacke is often called 'dirty sandstone' because it contains more than 15% clay matrix. It is a key rock for understanding tectonic settings and ancient deep-sea trench systems.

Field identification & locations

Identify in the field by its dark, uniform color, extreme toughness compared to other sandstones, and the absence of clear bedding planes in small specimens. Commonly found in coastal or mountainous regions.