Siltstone

Physical Properties

Hardness: 3-4 on Mohs scale; Color: Grey to brownish-grey; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained); Cleavage: None, usually breaks into irregular fragments.

Formation & Geological History

Formed from the lithification of silt-sized particles. These particles are typically deposited in low-energy environments like river floodplains, lagoons, or deep ocean basins. Age can range from modern to billions of years old depending on the strata.

Uses & Applications

Primarily used as fill material, in the manufacture of cement, or occasionally as low-quality building stone. It is not generally used in jewelry due to its softness and lack of aesthetic appeal.

Geological Facts

Siltstone is finer than sandstone but coarser than shale. Unlike shale, it lacks fissility (the ability to split into thin layers easily). It often preserves ripple marks or fossils within its fine layers.

Field Identification & Locations

Identify in the field by its gritty 'toothy' feel when rubbed against the teeth (unlike clay) and its lack of visible grains to the naked eye (unlike sand). It is common in sedimentary basins worldwide.

Identified on: 4/18/2026

Mode: Standard