sedimentary

Chert (and Chalcedony/Agate mixture)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, and white with translucent windows; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Gray, tan, and white with translucent windows
Luster: Waxy to vitreous

Identified March 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich fluids in limestone cavities or as deep-sea biochemical deposits. The specimen shows rounding from alluvial transport (river action) and may date back anywhere from 10 to 500 million years depending on the region.

Uses & applications

Used historically for stone tools and arrowheads; currently used as landscaping gravel, in concrete aggregate, and as a semi-precious gemstone when polished into agates.

Geological facts

Chert was the most important industrial mineral of the Stone Age due to its ability to hold a razor-sharp edge. It is virtually immune to chemical weathering, which is why it often appears as rounded pebbles in stream beds while other rocks disintegrate.

Field identification & locations

Identify in the field by its hardness (it will scratch glass and steel) and its shell-like (conchoidal) fracture pattern. Commonly found in river beds, beaches, and glacial till. Look for translucent edges when held to light.