Siltstone

Physical Properties

Hardness: 3-4 (Mohs scale); Color: Brownish-tan to grey; Luster: Dull/Earthy; Crystal structure: Clastic (microscopic grains); Cleavage: None, usually breaks in irregular blocks; Specific gravity: approx. 2.6

Formation & Geological History

Formed from the lithification of silt-sized particles deposited in low-energy environments like river floodplains, lakes, or deep-sea basins. These deposits are often millions of years old, typically dating from the Paleozoic to Cenozoic eras.

Uses & Applications

Used primarily as low-grade construction fill, road base, or flagstone. It lacks the durability of sandstone or granite and is rarely used for jewelry.

Geological Facts

Siltstone is often mistaken for shale, but it lacks the fissility (the ability to split into thin layers) that characterizes true shale. It feels slightly gritty against the teeth, unlike clay-based rocks.

Field Identification & Locations

Identify in the field by its fine-grained texture that is grittier than shale but smoother than sandstone. It is commonly found in sedimentary basins and near outcrops of ancient riverbeds. It has low value for collectors.

Identified on: 4/25/2026

Mode: Standard