sedimentary

Chalcedony (Agate Pebble)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Translucent tan to brownish-grey; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64

Hardness: 6
Color: Translucent tan to brownish-grey
Luster: Waxy to dull

Identified July 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the deposition of silica-rich groundwater in cavities of igneous or sedimentary rocks. This specific specimen appears to be a water-worn river or beach pebble, likely resulting from the erosion of larger host rocks.

Uses & applications

Used for lapidary work, jewelry (beads and cabochons), and as a collector's item for rockhounds. Historically used for making stone tools due to its hardness and sharp fracture edges.

Geological facts

Chalcedony is a cryptocrystalline form of silica. Agates are a variety of chalcedony that often show banding, though small water-worn pebbles like this may only show faint patterns or solid translucent colors.

Field identification & locations

Identify in the field by its translucency (hold it up to a light source), waxy luster when dry, and inability to be scratched by a steel knife. Commonly found in riverbeds, glacial till, and coastal areas.

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