sedimentary

Weathered chert or chalcedony pebble

Microcrystalline quartz (SiO₂), likely chert/chalcedony

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; opaque gray-brown to tan, with dull-to-waxy luster and conchoidal fracture. The rounded shape indicates water abrasion; white patches may be quartz, calcite, or surface mineral residue.

Identified September 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed when silica-rich groundwater replaced or cemented sediment, commonly in limestone or volcanic environments; the exact geologic age cannot be determined from the image. Later erosion produced the smooth pebble.

Uses & applications

Mostly a collecting or decorative specimen; chert can be used for flint tools, aggregate, and landscaping, while chalcedony varieties are used in cabochons and carvings.

Geological facts

Chert is composed of microscopic quartz crystals and may preserve fossils or show subtle banding. Its strong resistance to weathering allows it to remain as stream and beach pebbles after softer rocks erode.

Field identification & locations

Test cautiously with a steel file or knife: true chert/chalcedony resists scratching, whereas calcite-rich material scratches easily. Common in gravel beds, river channels, beaches, and sedimentary terrains worldwide; vinegar testing the white patch may indicate calcite if it effervesces.