Siltstone
Siltstone is a fine-grained sedimentary rock made mostly of silt-sized particles. It feels smoother than sandstone but grittier than claystone or shale.
Siltstone properties at a glance
| Type | Sedimentary Rocks |
|---|---|
| Chemical formula | Variable mineral composition |
| Mohs hardness | 3.5 |
| Luster | dull |
| Streak | Variable; often light gray to brown |
| Cleavage | Variable by mineral grains |
| Fracture | Uneven to blocky |
| Specific gravity | 2.45 |
| Transparency | opaque |
| Common colors | Gray, Tan, Brown, Red, Green |
| Rarity | common |
Geology & background
Forms when silt-sized sediment is deposited in quiet water or low-energy settings and later compacted and cemented into rock.
Siltstone records low-energy rivers, floodplains, lakes, deltas, and shallow marine settings.
How to identify Siltstone
- Look for a fine, dull rock that is gritty but not sandy.
- Siltstone is usually less fissile than shale.
- Common colors include gray, tan, brown, red, and green.
- Fresh broken edges can reveal subtle bedding or lamination.
Where Siltstone is found
Siltstone occurs in many sedimentary settings worldwide; exact locality should be verified from field notes, maps, or specimen provenance.
How much is Siltstone worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Siltstone is usually valued for teaching, field context, locality, texture, fossils, color, or unusual structures rather than high gem value.
Common uses
- Field identification
- Education
- Depositional environment study
- Construction fill or aggregate when suitable
Interesting facts
- Siltstone grain size sits between sandstone and mudstone.
- It may not split into thin sheets as easily as shale.
- A rough fresh surface can feel slightly gritty against teeth or a fingernail.
Care & safety
Siltstone can be soft, porous, crumbly, or weakly cemented depending on grain size, cement, and weathering.
Avoid inhaling dust. Do not cut, grind, or polish unknown sedimentary rocks without eye protection and a properly rated respirator.
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