Sandstone (River Cobble)

Physical Properties

Hardness: 6-7 (Mohs); Color: Tan/Beige/Yellowish-brown; Luster: Dull/Earthy; Crystal Structure: Clastic; Cleavage: None; Specific Gravity: 2.2-2.8. Highly abrasive texture.

Formation & Geological History

Formed from the accumulation and cementation of sand-sized grains, primarily quartz. This specific specimen has been rounded by fluvial processes (river erosion) over thousands of years.

Uses & Applications

Commonly used in landscaping as decorative stone, construction aggregate, and historically for grinding tools or building materials.

Geological Facts

Sandstone makes up about 20% of all sedimentary rocks on Earth. The smooth, oval shape is the result of 'saltation' where the rock bounces along a riverbed, wearing down its edges.

Field Identification & Locations

Identify by its gritty, sandpaper-like feel and visible small grains. Common in riverbeds, deltas, and landscaping mulch beds. To test, it should be able to scratch glass if it has high quartz content.

Identified on: 4/19/2026

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