mineral

Chalcedony (River Agate)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Tans, grays, and yellowish-browns with waxy translucency; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline (Trigonal/Monoclinic); No cleavage.

Hardness: 6
Color: Tans, grays, and yellowish-browns with waxy translucency
Luster: Waxy to vitreous

Identified July 6, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Tans, grays, and yellowish-browns with waxy translucency; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline (Trigonal/Monoclinic); No cleavage.

Formation & geological history

Formed by the deposition of silica from groundwater in cavities of igneous or sedimentary rocks. This specimen shows significant rounding and frosting from fluvial (river) transport and water erosion.

Uses & applications

Commonly used for lapidary work (tumbled stones, beads, cabochons) and as a decorative landscape gravel or collector's item.

Geological facts

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. When it shows banding, it is called agate; when solid or translucent like this, it is basic chalcedony.

Field identification & locations

Look for a translucent, waxy appearance when held up to a light source. It often has 'chatter marks' or small crescent-shaped impacts on the surface from hitting other rocks in a stream bed.