sedimentary

Chert (likely chalcedony-rich flint)

Microcrystalline quartz, SiO₂ (quartz/chalcedony)

AI-generated identification · may be incorrect

Typically Mohs 6.5–7, opaque to translucent gray-brown, with waxy to dull luster and no visible cleavage; conchoidal fracture and dense, fine-grained texture are characteristic. The specimen appears weathered with a pale silica-rich interior and brown iron-oxide staining.

Identified September 6, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms by replacement or accumulation of microcrystalline silica in sedimentary settings, commonly from biogenic silica or groundwater alteration. Its exact geological age cannot be determined from the photograph; chert occurs from Precambrian strata to modern deposits.

Uses & applications

Historically important for sharp tools and fire-starting; today used as aggregate, lapidary material, and a common collectible. Attractive translucent pieces may be polished, though this example is modest grade.

Geological facts

Chert is not a single mineral but a rock dominated by quartz varieties, especially chalcedony and microcrystalline quartz. Flint is generally a dark, high-quality chert traditionally associated with chalk formations.

Field identification & locations

Identify it by testing for glassy/conchoidal fracture, high hardness, and inability to scratch with a steel knife easily; it commonly occurs as nodules or beds in limestone, chalk, and volcanic-sedimentary rocks. Common sources include the Midwest and Great Plains, Texas, Europe, and many worldwide sedimentary basins.