Chalcedony (Agate/Chert)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Translucent tan, white, and waxy yellow; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.64.

Formation & Geological History

Formed from the deposition of silica from groundwater in cavities of igneous rocks or as sedimentary nodules. This specimen appears to be a seam agate or a fragment of a larger nodule.

Uses & Applications

Used primarily for lapidary work, jewelry (cabochons), collection, and historically for making stone tools due to its sharp fractured edges.

Geological Facts

Chalcedony is a cryptocrystalline form of silica, meaning its crystals are too small to be seen under a normal microscope. Agate is a variety specifically known for its translucency and patterns.

Field Identification & Locations

Identify by its waxy luster, lack of visible crystals, and conchoidal (shell-like) fracture patterns. Often found in riverbeds or volcanic regions common in the western United States and Brazil.

Identified on: 5/2/2026

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