sedimentary
Graywacke (Sandstone)
Graywacke Sandstone
Hardness: 6-7 on Mohs scale; Color: Dark gray, greenish-gray, or black; Luster: Dull to earthy; Crystal structure: Clastic (sand-sized grains in a finer matrix); Cleavage: None; Specific gravity: 2.6-2.7
- Hardness
- 6-7 on Mohs scale
- Color
- Dark gray, greenish-gray, or black
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark gray, greenish-gray, or black; Luster: Dull to earthy; Crystal structure: Clastic (sand-sized grains in a finer matrix); Cleavage: None; Specific gravity: 2.6-2.7
Formation & geological history
Formed from the rapid deposition of poorly sorted sediment in deep-ocean environments, often associated with submarine landslides or turbidity currents. Most date from the Paleozoic or Mesozoic eras.
Uses & applications
Used primarily as road base, railroad ballast, and high-quality construction aggregate due to its hardness and durability. Occasionally used for decorative stone and carving.
Geological facts
Often called 'dirty sandstone' because of its high clay content and poor sorting. It is a key rock type for understanding ancient plate tectonic movements and deep-sea turbidite systems.
Field identification & locations
Identify by its gritty texture, extreme hardness (will scratch glass), and 'salt and pepper' appearance of quartz and feldspar grains in a dark matrix. Common in mountain belts and coastal regions.
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