sedimentary
Greenstone / Graywacke
Lithic Sandstone (Graywacke)
Hardness: 6-7 on Mohs scale; Color: Dull gray-green to dark gray; Luster: Dull or earthy; Crystal structure: Clastic (fine-grained); Specific gravity: 2.6-2.7. It has a rough, gritty texture and lacks visible bedding planes in small river-worn cobbles.
- Hardness
- 6-7 on Mohs scale
- Color
- Dull gray-green to dark gray
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dull gray-green to dark gray; Luster: Dull or earthy; Crystal structure: Clastic (fine-grained); Specific gravity: 2.6-2.7. It has a rough, gritty texture and lacks visible bedding planes in small river-worn cobbles.
Formation & geological history
Formed in deep marine environments from the rapid deposition of poorly sorted sediment and volcanic ash, often in submarine fans. Most graywacke deposits date from the Paleozoic or Mesozoic eras and have undergone low-grade burial metamorphism.
Uses & applications
Commonly used as crushed stone for road construction, railroad ballast, and aggregate in concrete. High-quality specimens are sometimes used as decorative river rocks in landscaping.
Geological facts
Graywacke is often referred to as a 'dirty sandstone' because of its high clay matrix content. It is a signature rock of subduction zones and mountain-building regions like the Franciscan Complex in California or the Southern Alps of New Zealand.
Field identification & locations
Identify in the field by its distinct greenish-gray hue, lack of clear layers, and its ability to scratch glass (due to quartz content). It is commonly found in gravel bars of rivers draining coastal mountain ranges.
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