sedimentary
Brown Chert or Ironstone
Cryptocrystalline Quartz (SiO2) with Iron Oxides
Hardness: 6.5-7 on Mohs scale; Color: Rusty brown, tan, ochre; Luster: Waxy to dull; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Rusty brown, tan, ochre
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Rusty brown, tan, ochre; Luster: Waxy to dull; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed via the accumulation of silica-rich fluids in sedimentary basins or as nodules within limestone/chalk, often enriched with limonite or hematite staining. Geological age varies widely by location.
Uses & applications
Historically used for stone tools (flint knapping) due to sharp edges; currently used for landscaping, aggregate in construction, and occasionally for lapidary work.
Geological facts
Chert is essentially the same material as flint, but usually refers to lighter or earth-toned specimens. It is so tough it was once used as 'strike-a-lights' to start fires.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Commonly found in riverbeds and limestone outcrops.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Brecciated Chert (Microcrystalline Silica)
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