mineral

Chalcedony (Agate/Seashell replacement)

Silicon Dioxide (SiO2) - Cryptocrystalline Quartz

Hardness: 6.5-7 (Mohs), Color: Translucent white to tan, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microscopic), Cleavage: None/Conchoidal fracture

Hardness: 6

Identified May 25, 2026More mineral →

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Physical properties

Hardness: 6.5-7 (Mohs), Color: Translucent white to tan, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microscopic), Cleavage: None/Conchoidal fracture

Formation & geological history

Formed by the precipitation of silica from groundwater within cavities, often replacing biological matter like shells in sedimentary environments near volcanic activity.

Uses & applications

Used in gemstone carving, jewelry, as decorative collector specimens, and historically for tools due to conchoidal fracturing.

Geological facts

This specimen appears to be a 'chalcedony coral' or a silica-replaced shell fragment, where quartz has slowly replaced the original calcium carbonate structure atom by atom.

Field identification & locations

Identify by its waxy luster, translucency when held to light, and its inability to be scratched by a steel knife. Commonly found in riverbeds and coastal regions with ancient limestone or volcanic deposits.

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