sedimentary
Chert (often called Flint or Jasper locally)
Chert (Microcrystalline Quartz, Formula: SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brown, or grayish-yellow; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (features prominent conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, or grayish-yellow
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, or grayish-yellow; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (features prominent conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica in sedimentary environments, often replacing carbonate materials in limestone or through the accumulation of siliceous organic remains (like diatoms or radiolarians). It can form as nodules or bedded layers across various geological periods from the Precambrian to the present.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) and to start fires (flint and steel). Currently used in road construction, as an abrasive, and occasionally in lapidary work for jewelry or decor.
Geological facts
Chert is so hard it can scratch steel. It breaks with a conchoidal fracture, creating extremely sharp edges, which is why it was the primary material for tool-making in the Stone Age. Depending on color, it is frequently referred to as flint (black/dark grey) or jasper (red/brown).
Field identification & locations
Identify by its 'shell-like' curved fractures (conchoidal), waxy feel, and extreme hardness. Commonly found in limestone outcrops, riverbeds, and glacial till. Collectors look for unique colors or specimens that show signs of ancient human working (lithic debitage).
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