sedimentary
Chert (and Jasper/Chalcedony mixture)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: brown, tan, creamy white, and gray. Luster: waxy to vitreous. Crystal structure: Trigonal (microcrystalline). Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
- Hardness
- 6
- Color
- brown, tan, creamy white, and gray
- Luster
- waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: brown, tan, creamy white, and gray. Luster: waxy to vitreous. Crystal structure: Trigonal (microcrystalline). Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
Formation & geological history
Formed by the accumulation of siliceous remains of organisms like diatoms or radiolarians, or through chemical precipitation in hydrothermal or sedimentary environments in marine or volcanic settings.
Uses & applications
Used historically for stone tools (arrowheads). Modern uses include gemstones, landscaping, and industrial abrasives.
Geological facts
Chert is so hard it can produce sparks when struck against steel, which made it a critical survival material for early humans. Its colors are usually due to trace amounts of iron or manganese oxides.
Field identification & locations
Look for a glassy/waxy surface and sharp, curved (conchoidal) edges where the stone has chipped. It will scratch glass and cannot be scratched by a steel knife. Frequently found in riverbeds or within limestone layers.
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