sedimentary
Chert (Nodular)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Brown, tan, and gray; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- Brown, tan, and gray
- Luster
- Waxy to dull/earthy
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, tan, and gray; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the replacement of limestone or carbonate sediment by silica-rich fluids (secondary replacement) or by the accumulation of siliceous remains of organisms like diatoms and sponges. Common in Paleozoic and Mesozoic sedimentary strata.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative landscaping rock, in construction aggregate, and occasionally for lapidary work.
Geological facts
Chert is so hard that it can strike sparks against steel, a property used by ancient people to start fires. Flint is a high-quality variety of chert often associated with chalk.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel blade), smooth waxy texture on fresh breaks, and curved conchoidal fracture patterns. Found in riverbeds and limestone outcrops.
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