mineral

Agate Geode

Silicon Dioxide (SiO2) - Variety Agate with Quartz crystal center

Hardness: 7 (Mohs); Color: Banded grey/black outer layer with translucent/white crystals; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microcrystalline agate/macrocrystalline quartz); Cleavage: None; Specific Gravity: 2.60-2.65.

Hardness: 7 (Mohs)
Luster: Vitreous to waxy

Identified June 26, 2026More mineral →

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Physical properties

Formation & geological history

Formed in volcanic rock cavities (vesicles) where silica-rich groundwater deposited layers of chalcedony over time, followed by larger quartz crystals in the remaining void. Typically dates from thousands to millions of years old depending on the host formation.

Uses & applications

Used primarily in lapidary work, jewelry, home decor, and as a popular collector's specimen. In industry, high-purity quartz is used for abrasive and optical purposes.

Geological facts

Geodes are often referred to as 'thunder eggs' in some regions; they appear as ordinary round stones on the outside, hiding their crystalline interior until cracked open.

Field identification & locations

Identify in the field by looking for rounded, knobby stones that feel lighter than their size suggests. Common in Brazil, Uruguay, and volcanic regions of the Western USA. Look for the distinct concentric banding around the hollow center.

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Mineral/Rock

Banded Gneiss (River Rock/Pebble)

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Metamorphic Rock

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mineral