sedimentary
Chert (and Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 7.0 (Mohs); Color: Gray, tan, or white with waxy luster; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7
- Color
- Gray, tan, or white with waxy luster
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Physical properties
Hardness: 7.0 (Mohs); Color: Gray, tan, or white with waxy luster; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary basins. Often occurs as nodules in limestone or bedded deposits.
Uses & applications
Historically used for stone tools and arrowheads; used industrially as a silica source, in construction aggregate, and occasionally in lapidary work.
Geological facts
Chert has a conchoidal fracture which means it breaks into very sharp edges, similar to glass, making it the primary material for prehistoric survival tools.
Field identification & locations
Identify by its waxy texture and ability to scratch glass. Commonly found in limestone outcrops, river beds, and glacial till. Collectors look for unique banding or vugs.
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