sedimentary (secondary mineral formation)

Agate Geode

Chalcedony (SiO2 - Silicon Dioxide)

Hardness: 6.5-7 on Mohs scale. Color: Translucent grey, white banding, with clear microcrystalline quartz interior. Luster: Vitreous to waxy. Structure: Cryptocrystalline banding with a druzy crystal cavity. Cleavage: None (conchoidal fracture). Specific Gravity: 2.58–2.64.

Hardness: 6
Luster: Vitreous to waxy

Identified January 10, 2026More sedimentary (secondary mineral formation) →

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Physical properties

Formation & geological history

Formed by the deposition of silica-rich groundwater into cavities (vesicles) within igneous or sedimentary rocks. Over millions of years, layers of chalcedony build up on the cavity walls, often leaving a hollow center lined with crystals. These can range from millions to hundreds of millions of years in age.

Uses & applications

Commonly used in jewelry (cabochons), lapidary art, and as decorative collector specimens. Industrially, its hardness and chemical resistance make it useful for mortars and pestles or burnishing tools.

Geological facts

The word 'agate' is derived from the Achates River in Sicily, where agates were first discovered in ancient times. Unlike solid agates, geodes are defined by their hollow, crystal-lined centers. This specific specimen shows 'druzy' quartz, which refers to the tiny, glittering crystals in the center.

Field identification & locations

Identify in the field by looking for rounded, potato-like nodules that feel lighter than solid rocks. When broken or cut, they reveal internal banding and a hollow crystal center. Common locations include Brazil, Uruguay, Mexico, and the United States (notably Oregon, Iowa, and Lake Superior regions).

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