sedimentary

Siltstone

Siltstone (clastic sedimentary rock)

Hardness: 3-5 (Mohs scale); Color: Tan, beige, light brown; Luster: Dull, earthy; Crystal structure: Non-crystalline clastic; Cleavage: None/Planar (fissility); Specific gravity: 2.1-2.6

Hardness: 3-5 (Mohs scale)
Color: Tan, beige, light brown
Luster: Dull, earthy

Identified July 3, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (Mohs scale); Color: Tan, beige, light brown; Luster: Dull, earthy; Crystal structure: Non-crystalline clastic; Cleavage: None/Planar (fissility); Specific gravity: 2.1-2.6

Formation & geological history

Formed from the lithification of silt-sized particles deposited in low-energy environments such as river floodplains, lake beds, or marine shelves. These particles are smaller than sand but larger than clay.

Uses & applications

Used primarily in construction as fill material or road base. Occasionally used for exterior landscaping or as a budget-friendly flagstone when well-cemented.

Geological facts

Siltstone differs from shale by its lack of fissility (thin layer splitting) and from sandstone by its significantly smoother texture to the touch.

Field identification & locations

Identify in the field by its fine-grained texture that feels slightly gritty against the teeth but smooth to the fingers. Common in sedimentary basins worldwide.