sedimentary

Riverstone (Siltstone/Mudstone)

Siltstone

Hardness: 3-4 (Mohs scale); Color: Dull gray, tan, or greenish-gray; Luster: Dull/Earthy; Crystal structure: Amorphous/Fine-grained; Cleavage: None/Irregular; Specific gravity: 2.6

Hardness: 3-4 (Mohs scale)
Color: Dull gray, tan, or greenish-gray
Luster: Dull/Earthy

Identified June 30, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs scale); Color: Dull gray, tan, or greenish-gray; Luster: Dull/Earthy; Crystal structure: Amorphous/Fine-grained; Cleavage: None/Irregular; Specific gravity: 2.6

Formation & geological history

Formed from the lithification of silt-sized particles (larger than clay but smaller than sand) deposited in low-energy aquatic environments like floodplains or riverbeds. Often Paleozoic to Cenozoic in age.

Uses & applications

Used primarily as decorative landscaping stones, filler material in construction, or as smooth 'worry stones' for handling.

Geological facts

River stones develop their characteristic smooth, rounded shape through the process of saltation and attrition—being tumbled by water against other rocks over thousands of years.

Field identification & locations

Identify in the field by its smooth, water-worn texture and fine grain that is too small to see with the naked eye. Commonly found in current or ancient riverbeds and shorelines.