sedimentary

Greywacke (River Slikkenstone)

Greywacke (Impure Sandstone)

Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal with hints of green or brown; Luster: Dull to earthy; Texture: Fine to medium-grained; Structure: Massive, lacking visible bedding in small fragments.

Hardness: 6-7 on Mohs scale
Color: Dark grey to charcoal with hints of green or brown
Luster: Dull to earthy

Identified March 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in deep ocean environments (turbidites) where underwater landslides deposit a mix of sand, silt, and clay. Most specimens date from the Paleozoic or Mesozoic eras. This specific specimen has been shaped and smoothed by fluvial or glacial transport.

Uses & applications

Primarily used as construction aggregate, road ballast, and in the production of high-strength concrete. Occasionally used as a decorative stone in landscaping.

Geological facts

Greywacke is known for its 'immature' mineral composition, meaning the grains have not been rounded or sorted by long-term weathering, suggesting rapid deposition. It is a key rock type in many mountain belts like the Appalachians and the Alps.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its dark, 'dirty' appearance compared to pure sandstone. Commonly found in riverbeds near mountainous terrain and along ancient plate boundaries.

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