sedimentary

Geode (Unopened)

Siliceous Concretion / Geode

Hardness: 6.5-7 (outer shell); Color: Tan, brown, or grayish exterior; Luster: Dull, earthy exterior; Structure: Spherical or subspherical with a lumpy, cauliflower-like texture; Specific Gravity: 2.6-2.7 (often feels lighter than solid rock if hollow).

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

Identified April 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary rock cavities (typically limestone) via the precipitation of minerals (usually silica) from hydrothermal fluids or groundwater over thousands of years.

Uses & applications

Primarily used as collector specimens, educational items, and for lapidary work (cracking open to reveal internal crystals).

Geological facts

Geodes are often 'hidden gems'—until you crack them open, you don't know if the interior contains clear quartz, amethyst, calcite, or even rare minerals. Some large geodes can be over several meters in diameter.

Field identification & locations

Identify in the field by looking for rounded, nodular rocks that appear out of place in layered limestone. These often feel lighter than regular rocks because they are partially hollow. Common in states like Iowa (Keokuk), Indiana, and Arizona.