Greywacke

Physical Properties

Hardness: 6.0-7.0 (Mohs scale); Color: Dark grey, greenish-grey, or bluish-black; Luster: Dull or earthy; Crystal structure: Clastic (composed of grains); Cleavage: None (conchoidal to irregular fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed in deep-ocean trenches and submarine canyons through turbidity currents (underwater landslides). Characterized by poor sorting and high matrix content, typically dating from the Paleozoic or Mesozoic eras.

Uses & Applications

Used primarily as aggregate for construction, road base, railway ballast, and occasionally for sea defenses or decorative stone in paving.

Geological Facts

Greywacke is often called 'dirty sandstone' because of its high clay content. It is a common component of the bedrock in New Zealand, the British Isles, and the Appalachian mountains.

Field Identification & Locations

Identify in the field by its dark color, extreme hardness (it will scratch glass), and visible angular grains of quartz and feldspar embedded in a fine-grained matrix. It often shows thin white quartz veining as seen in this specimen.

Identified on: 4/29/2026

Mode: Standard