sedimentary
Chert (Common Opal/Jasper variety)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, smoky grey, tan; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Brown, smoky grey, tan
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, smoky grey, tan; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation from hydrothermal fluids in limestone/dolomite cavities. This specimen likely formed in a volcanic or sedimentary basin.
Uses & applications
Historical use for stone tools and arrowheads; modern use in landscaping, lapidary (cabochons), and as a gravel component.
Geological facts
Chert is one of the most durable rocks on the planet. Its ability to break with sharp, predictable edges made it the primary material for human tools for over two million years.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture, waxy surface, and extreme hardness (it will easily scratch glass). Often found in riverbeds or as nodules within limestone outcrops.
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