mineral

River Chalcedony / Agate

Chalcedony (Silicon Dioxide, SiO2)

Hardness: 6.5-7 on the Mohs scale. Color: Translucent grey to brown with waxy luster. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.64.

Hardness: 6
Color: Translucent grey to brown with waxy luster

Identified March 24, 2026More mineral →

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Physical properties

Hardness: 6.5-7 on the Mohs scale. Color: Translucent grey to brown with waxy luster. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.64.

Formation & geological history

Formed as a chemical precipitate from silica-rich groundwater within volcanic cavities (vesicles) or sedimentary fissures. Often millions of years old, it is weathered out of its host rock and polished naturally by river motion.

Uses & applications

Used primarily in lapidary work for making cabochons, beads, and jewelry. Also used for decorative 'river rocks' in landscaping and as a popular specimen for rock tumbling.

Geological facts

Chalcedony is a broad term for microcrystalline quartz. When it exhibits banding, it is called Agate. Because it is highly resistant to weathering, it often survives long trips downstream, becoming rounded and smooth over centuries.

Field identification & locations

Identify in the field by its translucency (light can pass through it when held to a torch), smooth waxy feel when wet, and lack of visible grains. Common in pebble beaches and gravel bars of ancient riverbeds.

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