sedimentary
Chert (Chalcedony/Agate)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Honey-yellow to orange-brown (due to iron oxides); Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.
- Hardness
- 7 on Mohs scale
- Color
- Honey-yellow to orange-brown (due to iron oxides)
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Honey-yellow to orange-brown (due to iron oxides); Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.
Formation & geological history
Formed by the precipitation of silica from groundwater or within sedimentary rock layers, often replacing original mineral material or filling cavities. It can range from millions to hundreds of millions of years old.
Uses & applications
Historically used for making stone tools; currently used for lapidary work, jewelry (as agate/carnelian), and as decorative gravel.
Geological facts
This specimen displays a classic conchoidal fracture (shell-like curves), which is why similar materials like flint were prized by prehistoric humans for making sharp arrowheads and knives.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and the way it breaks into sharp, curved edges. Commonly found in riverbeds or within limestone formations.
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