sedimentary
Chert (specifically Flint or Common Chert)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Mottle light gray, white, tan, and rusty brown; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Mottle light gray, white, tan, and rusty brown
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Mottle light gray, white, tan, and rusty brown; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by chemical precipitation in marine or groundwater environments. This specimen shows a weathered exterior (cortex) with a fresher interiors revealed by fractures.
Uses & applications
Historically used for stone tools (arrowheads/knives) due to sharp edges; currently used as road aggregate, decorative masonry, and in some ceramics.
Geological facts
Chert is so hard it can strike sparks against steel to start fires. It is one of the most common materials used by early humans for tool making due to its predictable conchoidal fracturing.
Field identification & locations
Identify by its waxy texture, extreme hardness (it will scratch glass), and shell-like (conchoidal) fracture patterns. Look for 'nodes' or nodules in limestone or chalk beds.
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