sedimentary
Chert (Cherty Limestone / Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically gray, tan, or brown with waxy luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- typically gray, tan, or brown with waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: typically gray, tan, or brown with waxy luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica from seawater or the replacement of carbonate minerals (like limestone) by silica-rich fluids. Often found as nodules or layers in marine sedimentary rocks dating from the Precambrian to Holocene.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp fracture edges. Industrially used as decorative stone, road ballast, or in the manufacturing of glass and ceramics.
Geological facts
Chert is so hard it can scratch steel and was used for centuries as a 'strike-a-light' to create sparks for fire. Darker varieties are commonly known as Flint.
Field identification & locations
Identify in the field by its characteristic 'seashell-like' (conchoidal) fracture, sharp edges, and ability to scratch glass. It often feels smoother or waxier than the limestone it may be embedded in.
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