sedimentary

Agatized Chert / Chalcedony

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs scale), Color: tan, yellow, and translucent white, Luster: waxy to vitreous, Crystal Structure: Cryptocrystalline (Microcrystalline Quartz), Cleavage: None (has conchoidal fracture), Specific Gravity: 2.58-2.64

Hardness: 6

Identified March 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from groundwater into cavities or replacing organic matter in sedimentary rock layers. Often found in ancient riverbeds or limestone deposits where silica-rich fluids circulated millions of years ago.

Uses & applications

Commonly used for lapidary work (polishing and cabochons), primitive tool making (arrowheads/knapping), and as decorative landscaping stone.

Geological facts

This specimen shows clear 'banding' or swirls characteristic of agate, which is a translucent variety of chalcedony. The rough, pitted exterior is typical of river-worn stones where the harder silica mineral survives while the surrounding matrix wears away.

Field identification & locations

Identify in the field by its waxy texture, translucent edges when held to light, and characteristic 'conchoidal' (shell-like) chips on the corners. Found frequently in the American Midwest and Great Lakes region riverbeds.

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