sedimentary
Chert (Chalcedony/Agate)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Gray, bluish-gray, earthy brown; Luster: Waxy to vitreous; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65.
- Hardness
- 6
- Color
- Gray, bluish-gray, earthy brown
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Gray, bluish-gray, earthy brown; Luster: Waxy to vitreous; Crystal Structure: Trigonal (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65.
Formation & geological history
Formed primarily through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine or groundwater environments. This specimen appears water-worn, likely from a fluvial or glacial deposit.
Uses & applications
Historically used for making stone tools (arrowheads). Modern uses include tumbled stones for jewelry, aquarium decor, and as a component in construction aggregate.
Geological facts
Chert is so hard it will spark when struck against steel. It was one of the first minerals used by early humans due to its predictable, sharp fracturing patterns.
Field identification & locations
Identify in the field by its waxy luster, smoothness when wet, and curved (conchoidal) shell-like chips on the edges. Commonly found in river beds and gravel pits.
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