sedimentary

Siltstone

Siltstone (primarily SiO2 with various silicates)

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs scale); Color: Brown, tan, red; Luster: Dull or earthy; Crystal structure: Clastic (non-crystalline fine grains); Cleavage: None/Massive; Specific Gravity: 2.6

Hardness: 6-7 (Mohs scale)
Color: Brown, tan, red
Luster: Dull or earthy

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 6-7 (Mohs scale); Color: Brown, tan, red; Luster: Dull or earthy; Crystal structure: Clastic (non-crystalline fine grains); Cleavage: None/Massive; Specific Gravity: 2.6

Formation & geological history

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

Uses & applications

Primarily used in construction as fill material or low-quality building stone; occasionally used as a decorative stone in landscaping or as carving material for low-stress items.

Geological facts

Siltstone is often mistaken for shale, but it lacks the characteristic fissility (splitting into thin layers) that shale possesses. It represents a transitional phase between sandstone and shale.

Field identification & locations

Identify in the field by its fine-grained texture that feels slightly gritty on the teeth (unlike clay) but smoother than sandstone. Found worldwide in sedimentary basins. Collectors should look for trace fossils often preserved in these sediments.