sedimentary
Chert (and Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale; Color: typically white, tan, gray, or brown; Luster: waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- typically white, tan, gray, or brown
- Luster
- waxy to dull
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Physical properties
Hardness: 6.5–7 on Mohs scale; Color: typically white, tan, gray, or brown; Luster: waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical precipitation of silica in groundwater. It can range in age from Precambrian to Cenozoic, often found as nodules in limestone or as bedded deposits.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Modern uses include glassmaking, abrasives, road construction aggregate, and occasionally as semi-precious gemstones when colorful (Jasper/Agate).
Geological facts
Chert was one of the first materials used by early humans for fire-starting and weaponry. It is so tough and chemically stable that it often remains behind as 'lag' on the landscape long after the surrounding limestone has weathered away.
Field identification & locations
Identify by its waxy texture, extreme hardness (will scratch glass), and 'clamshell-like' conchoidal fractures. Commonly found in limestone outcrops or as river cobbles. Collectors should look for sharp edges and a smooth, almost plastic-like feel on fresh surfaces.
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