sedimentary

Chert (Common Opal or Agate)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified July 17, 2026More sedimentary →

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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.