sedimentary
Graywacke Pebble with Quartz Vein
Argillaceous Sandstone (Graywacke)
Hardness: 6-7 (Mohs scale), Color: Dark gray/charcoal with white veins, Luster: Dull/Earthy, Crystal Structure: Clastic (fine-grained matrix), Specific Gravity: 2.6-2.7. Smooth, water-worn texture.
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark gray/charcoal with white veins, Luster: Dull/Earthy, Crystal Structure: Clastic (fine-grained matrix), Specific Gravity: 2.6-2.7. Smooth, water-worn texture.
Formation & geological history
Formed from the rapid deposition of muddy sand in deep ocean trenches or submarine fans. The white veins are secondary quartz or calcite deposits that filled cracks in the rock during tectonic activity.
Uses & applications
Used primarily as crushed stone for road base, railway ballast, and construction aggregate. Polished pebbles are often used in decorative landscaping or as worry stones.
Geological facts
Graywacke is often called 'dirty sandstone' because it contains a mix of poorly sorted grains and clay. The thin white lines are known as 'healed fractures' where mineral-rich water crystallized over millions of years.
Field identification & locations
Identify by its characteristic dark, uniform gray color and 'salt-and-pepper' texture under a lens. Found commonly in riverbeds and on beaches near mountainous/tectonically active coastlines.
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