sedimentary
Chert (Common Opal or Agate)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan, cream, and brown waxy banding, Luster: waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture)
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan, cream, and brown waxy banding, Luster: waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture)
Formation & geological history
Formed via the precipitation of silica from groundwater in sedimentary host rocks or volcanic ash beds. It often replaces original organic material or fills voids in rocks.
Uses & applications
Historically used for stone tools (arrowheads). Modern uses include landscaping, tumbling for collectors, and occasional use in jewelry as cabochons.
Geological facts
Chert is so durable that it is often found as smooth river pebbles long after the surrounding rock has eroded away. Specimens with distinct banding like this are often referred to as 'cold water agates' in some regions.
Field identification & locations
Identify by its waxy luster, smooth texture, and inability to be scratched by a steel knife. Common in gravel pits, riverbeds, and limestone outcrops worldwide.
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