sedimentary
Chert (Chalcedony/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, reddish, and grey; Luster: Waxy to vitreous (especially when wet); Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 6
- Color
- Brown, tan, reddish, and grey
- Luster
- Waxy to vitreous (especially when wet)
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, reddish, and grey; Luster: Waxy to vitreous (especially when wet); Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed primarily through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or by chemical precipitation from silica-rich fluids within prehistoric sedimentary basins. It is often found as nodules in limestone or as distinct bedded deposits.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers) due to its sharp edges when fractured. Currently used as decorative landscaping rock, in lapidary for polishing, and as an industrial abrasive.
Geological facts
Chert is essentially the same chemical composition as quartz but has an extremely fine-grained crystal structure. When it shows colorful banding, it is often called Agate; when it is dark and opaque, it is called Flint.
Field identification & locations
Identify in the field by its waxy texture and conchoidal (shell-like) fracture patterns. It is extremely common in riverbeds and gravel pits. It won't be scratched by a steel knife, which differentiates it from softer limestone.
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