sedimentary
Chert (Chalcedony/Agate)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, orange-brown with white patination; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Color
- Tan, orange-brown with white patination
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, orange-brown with white patination; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed via the accumulation of silica-rich biogenic material or chemical precipitation from groundwater within sedimentary host rocks. Common in ancient limestone and chalk beds.
Uses & applications
Historically used for stone tools due to sharp edges; currently used for lapidary work, jewelry (especially agates), and as a collection specimen.
Geological facts
Small fragments like this are often called 'river agates' or chert nodding. It is chemically identical to quartz but has a different microscopic structure. Many ancient arrowheads were crafted from this specific type of material.
Field identification & locations
Identify by its hardness (will scratch glass), waxy luster when broken, and lack of visible grains. Often found in gravel pits or streambeds where harder minerals accumulate.
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