mineral

Chalcedony Geode / Agate Nodule

Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: waxy white/gray interior with brownish exterior; Luster: waxy to vitreous; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None; Conchoidal fracture; Specific gravity: approx. 2.6

Hardness: 6
Color: waxy white/gray interior with brownish exterior
Luster: waxy to vitreous

Identified June 21, 2026More mineral →

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Physical properties

Formation & geological history

Formed by the deposition of silica from groundwater in cavities or vesicles within volcanic rocks or sedimentary formations. These typically form over thousands of years as mineral-rich water seeps into gas bubbles in cooling lava.

Uses & applications

Used primarily for lapidary work, jewelry (cabochons), collection specimens, and decorative carvings. It is also used as an industrial abrasive in some contexts.

Geological facts

Chalcedony was highly valued in antiquity for seal stones because hot wax does not stick to it. Many agates and chalcedony nodules are millions of years old, dating back to various volcanic eras.

Field identification & locations

Identify in the field by looking for rounded, rough-textured nodules that feel heavier than they look and show a translucent, waxy interior when broken. Commonly found in areas with past volcanic activity or in sedimentary gravels.

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