sedimentary
Chert (Chalcedony/Common Opal)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, translucent; Luster: Waxy to vitreous; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 6
- Color
- Brown, tan, translucent
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, translucent; Luster: Waxy to vitreous; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed by the precipitation of silica from groundwater or as a replacement of organic material or limestone. Found in nodules or layered beds in various sedimentary environments worldwide.
Uses & applications
Historically used for stone tools and weapons due to its sharp edges when fractured. Modern uses include abrasives, gravel, and minor lapidary use for jewelry ornaments.
Geological facts
Flint is a variety of chert. Because of its hardness and silica content, it was the primary material for 'fire-starting' technology and projectile points for thousands of years.
Field identification & locations
Identify by its characteristic waxy surface and conchoidal (shell-like) fracture patterns. It cannot be scratched by a steel knife. Common in riverbeds and limestone outcrops.
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