sedimentary
Chert (containing Orbicular/Agate patterns)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, cream, and reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Tan, cream, and reddish-brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, cream, and reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed by the precipitation of silica from groundwater or within deep-sea sedimentary environments. The specimens shown appear to have undergone secondary replacement processes, often found in Paleozoic to Mesozoic limestone or dolomite beds.
Uses & applications
Used historically for stone tools due to sharp fracture edges; currently used for lapidary work (cabochons), as an abrasive, and for decorative landscaping.
Geological facts
Chert is so hard it can create sparks when struck against steel, making it a primary tool for early fire-starting. The patterns in this specific piece suggest orbital or nodular growth during silica enrichment.
Field identification & locations
Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and the inability to be scratched by a steel knife. Common in the Midwest USA and mountainous limestone regions.
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