sedimentary
Chert / Flint Nodules
Chert (SiO2)
Hardness: 7 on Mohs scale; Color: typically grey, white, tan, or black with light-colored cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline; Fracture: conchoidal; Specific gravity: approx. 2.6
- Hardness
- 7 on Mohs scale
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically grey, white, tan, or black with light-colored cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline; Fracture: conchoidal; Specific gravity: approx. 2.6
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by chemical precipitation within limestone or chalk beds, often during the Cretaceous period.
Uses & applications
Historically used for stone tools (knapping) and fire-starting (flint); currently used in road construction, landscape gravel, and occasionally for lapidary work.
Geological facts
Flint is a variety of chert. This specimen shows a characteristic chalky outer layer or 'cortex' and the characteristic scallop-shaped conchoidal fractures common to high-silica rocks.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy inner texture, and smooth, curved fracture surfaces. Commonly found in chalk or limestone outcrops and along beaches near these formations.
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