sedimentary

Weathered chert or siliceous mudstone

Microcrystalline quartz (chalcedony), SiO₂; possibly silicified mudstone

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; gray-brown, opaque, earthy to waxy luster, very fine-grained with irregular conchoidal fractures. No visible cleavage; apparent angular cracking may reflect weathering or brecciation.

Identified August 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Likely formed when silica-rich groundwater replaced or cemented fine sediment, generally during diagenesis; the specimen’s exact geological age cannot be determined from the image. Common in sedimentary basins, nodules, and bedded chert formations.

Uses & applications

Crushed chert is used as aggregate and historically for cutting tools; attractive pieces are collected, polished, or used in lapidary work. This rough specimen has little commercial value.

Geological facts

Chert commonly preserves fossils or microscopic textures and can produce sharp edges when freshly fractured. The brown coloration likely comes from iron oxides or clay impurities.

Field identification & locations

Identify it by its very fine texture, high hardness, lack of visible grains, and inability to scratch with a steel knife easily; a streak test is usually white to pale gray. Common locations include limestone and other sedimentary terrains worldwide.