sedimentary
Chert (Tan/Brown variety)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or light brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or light brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or light 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 chemical precipitation from groundwater within limestone or chalk beds. Most specimens can range from Precambrian to recent ages.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative stone, in construction aggregate, and as a component in certain ceramics.
Geological facts
Chert is so hard it can scratch glass. When struck against steel, it can produce sparks, leading to its historical use as a fire-starter and in flintlock firearms.
Field identification & locations
Identify by its 'waxy' appearance, extreme hardness, and distinctive shell-like (conchoidal) curved fracture patterns. Commonly found in riverbeds or as nodules within limestone outcrops.
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