sedimentary
Chert (with small quartz inclusion)
Microcrystalline Silica (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: Gray, tan, and earthy brown with a smooth waxy luster in broken areas; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Gray, tan, and earthy brown with a smooth waxy luster in broken areas; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich organisms (like diatoms or radiolarians) or chemical precipitation in marine or groundwater environments. This process often replaces limestone or occurs as nodules in sedimentary strata over millions of years.
Uses & applications
Historically used for stone tools (flintknapping); currently used in road construction, as decorative landscaping stone, and occasionally in jewelry as common jasper or agate base.
Geological facts
Chert has been used by humans for hundreds of thousands of years to create arrowheads and scrapers because it breaks with incredibly sharp edges. It is chemically identical to quartz but has a different microscopic crystal structure.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, shell-like fracture pattern. Often found in riverbeds or as nodules within limestone outcrops. Collectors often look for examples with unique fossils or colorful banding.
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