sedimentary

Geode (Unopened)

Silica-rich Concretion (primarily SiO2)

Hardness: 6.5-7 (shell material); Color: Tan, brown, or gray exterior; Luster: Dull/earthy exterior; Structure: Spherical or subspherical nodule; Specific Gravity: Varies (hollow specimens are lighter).

Hardness: 6
Color: Tan, brown, or gray exterior
Luster: Dull/earthy exterior

Identified May 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of minerals into empty cavities in sedimentary rocks, such as limestone, or volcanic rocks. Over millions of years, mineral-rich groundwater deposits layers of crystals (usually quartz or calcite) inside the void.

Uses & applications

Used primarily for mineral collecting, interior decoration, and educational purposes. Once cracked open, they may reveal high-quality amethyst or quartz crystals suitable for jewelry or display.

Geological facts

The word 'geode' comes from the Greek word 'Geoides' meaning 'earth-like'. While the outside looks like an ordinary rock, the inside can contain beautiful crystalline structures that have been growing for millions of years.

Field identification & locations

Look for rounded, potato-shaped rocks that feel lighter than they look (indicating a hollow center). Common in areas with limestone bedrock such as the Midwestern United States (Indiana, Iowa, Illinois).

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Sandstone or Siltstone

Arenite (if sandstone)

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Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Sandstone

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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