sedimentary
Chert (and Quartz Gravel)
Chert (Cryptocrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: typically tan, brown, or gray (iron-stained); Luster: waxy to dull; Structure: microcrystalline (no visible grains); Cleavage: none (conchoidal fracture); SG: 2.65
- Hardness
- 7 on Mohs scale
- Color
- typically tan, brown, or gray (iron-stained)
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically tan, brown, or gray (iron-stained); Luster: waxy to dull; Structure: microcrystalline (no visible grains); Cleavage: none (conchoidal fracture); SG: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from groundwater. These specimens usually represent river-worn stones found in alluvial deposits or glacial till.
Uses & applications
Used primarily as road base, construction aggregate, and in landscaping. Historically used for making stone tools (arrowheads) due to its sharp edges when broken.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is one of the most common rocks found in riverbeds worldwide because it is highly resistant to chemical and physical weathering.
Field identification & locations
Identify in the field by its smooth, waxy texture and shell-like (conchoidal) fractures. It will not be scratched by a steel knife. Very common in limestone-rich regions and gravel pits.
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