sedimentary

Graywacke Pebble with Quartz Vein

Argillaceous Sandstone (Graywacke)

AI-generated identification · may be incorrect

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.

Identified June 25, 2026More sedimentary →

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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.