sedimentary
Quartz-veined Greywacke
Lithic Wacke with Siliceous Veins
Hardness: 6-7 (Mohs); Color: Medium grey with white bands; Luster: Dull/Earthy (matrix), Vitreous (veins); Texture: Granular matrix with crystalline veins; Cleavage: None; Specific Gravity: Approx. 2.6-2.7
- Hardness
- 6-7 (Mohs)
- Color
- Medium grey with white bands
- Luster
- Dull/Earthy (matrix), Vitreous (veins)
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Physical properties
Hardness: 6-7 (Mohs); Color: Medium grey with white bands; Luster: Dull/Earthy (matrix), Vitreous (veins); Texture: Granular matrix with crystalline veins; Cleavage: None; Specific Gravity: Approx. 2.6-2.7
Formation & geological history
Formed from the rapid deposition of muddy sands in deep-water marine environments (turbidity currents). The white veins formed later when tectonic stress caused fracturing, allowing silica-rich hydrothermal fluids to precipitate quartz in the cracks. Most specimens date from the Paleozoic or Mesozoic eras.
Uses & applications
Primarily used as crushed stone for road construction and concrete aggregate. Smooth, water-worn pebbles are often used in landscaping and as decorative garden stones.
Geological facts
Greywacke is often called 'dirty sandstone' because of its poorly sorted mixture of clay, quartz, and feldspar. The presence of white quartz veins is a classic indicator of past tectonic activity and fluid movement through the rock layer.
Field identification & locations
Identify in the field by its dark grey, tough, fine-grained matrix and the presence of hard, white quartz veins that cannot be scratched by a steel knife. Commonly found in coastal areas, riverbeds, and mountain ranges with ancient seafloor deposits.
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