mineral

Geode / Agate Nodule

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Greyish-brown exterior with purplish cast, often containing quartz, chalcedony, or amethyst inside; Luster: Dull or earthy exterior, vitreous inside; Structure: Spherical to subspherical concretion; Specific Gravity: ~2.65

Hardness: 6
Luster: Dull or earthy exterior, vitreous inside

Identified July 25, 2026More mineral →

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Physical properties

Formation & geological history

Forms in hollow cavities within volcanic rocks (lava flows) or sedimentary beds where mineral-rich water precipitates silica layers over long geological periods.

Uses & applications

Popular among lapidaries, mineral collectors, and jewelers. Unbroken geodes are often cracked open to reveal hidden quartz or amethyst crystals inside; polished halves are used as decorative displays or bookends.

Geological facts

Geodes look like ordinary, unremarkable rocks on the outside, but can contain stunning crystalline caverns on the inside. The outer shell is often durable chalcedony that survives weathering long after the surrounding matrix erodes.

Field identification & locations

Identify by its rounded shape, lightweight feel relative to its size (if hollow inside), and bumpy or chalcedonic outer rind. Commonly found in volcanic regions, deserts, and limestone beds (e.g., Midwest USA, Brazil, Mexico).