sedimentary
Greywacke (River/Beach Pebble)
Greywacke (Impure Sandstone)
Hardness: 6-7 (on Mohs scale); Color: Dull grey to greenish-grey; Luster: Dull or earthy; Crystal structure: Clastic (composed of poorly sorted angular grains of quartz, feldspar, and lithic fragments in a clay-rich matrix); Cleavage: None, breaks with an irregular fracture.
- Hardness
- 6-7 (on Mohs scale)
- Color
- Dull grey to greenish-grey
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 (on Mohs scale); Color: Dull grey to greenish-grey; Luster: Dull or earthy; Crystal structure: Clastic (composed of poorly sorted angular grains of quartz, feldspar, and lithic fragments in a clay-rich matrix); Cleavage: None, breaks with an irregular fracture.
Formation & geological history
Formed through the rapid deposition of muddy sands in deep-water environments, often via turbidity currents (underwater landslides). Usually associated with active tectonic margins and often dates back to the Paleozoic or Mesozoic eras.
Uses & applications
Commonly used as high-quality road aggregate, railway ballast, and in heavy construction due to its extreme hardness and resistance to weathering. Small, water-worn pebbles are often used in landscaping or as decorative stones.
Geological facts
Greywacke is sometimes called 'dirty sandstone' because of its high clay and silt content. It is unique among sandstones for containing fragments of other rocks (lithics) rather than just mineral grains.
Field identification & locations
Identify in the field by its characteristic 'pepper and salt' appearance when viewed closely, and its lack of obvious layering (unlike shale). It feels gritty to the touch and is significantly harder than limestone. It is frequently found as rounded cobbles in riverbeds and on rocky beaches.
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