sedimentary
Chert (specifically Flint or Common Opal fragment)
Cryptocrystalline Silica (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan, white, and waxy brown, Luster: Waxy to vitreous, Crystal Structure: Cryptocrystalline (microscopic quartz), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan, white, and waxy brown, Luster: Waxy to vitreous, Crystal Structure: Cryptocrystalline (microscopic quartz), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
Formation & geological history
Formed through the accumulation of siliceous remains of organisms or precipitation from silica-rich groundwater within limestone or chalk beds. Often associated with Cretaceous or older marine deposits.
Uses & applications
Historical use for stone tools (knapping) due to sharp edges; currently used for road surfacing, jewelry (if colorful), and as fire-starting flint.
Geological facts
Chert and flint were among the first materials used by prehistoric humans to create spearheads and knives because of their predictable conchoidal fracture which creates razor-sharp edges.
Field identification & locations
Identify by its waxy texture and shell-like (conchoidal) breakage patterns. It will easily scratch glass. Commonly found as nodules in limestone outcrops or as river-worn pebbles.
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