sedimentary

Geode (unopened)

Chert / Chalcedony Geode (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Dull brownish-grey exterior with possible white mineral patches; Luster: Dull/Earthy (exterior), Vitreous (interior when broken); Structure: Spherical/nodular with a hollow or crystal-filled center; Cleavage: None (conchoidal fracture on quartz).

Hardness: 6
Luster: Dull/Earthy (exterior), Vitreous (interior when broken)

Identified April 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of minerals from water inside hollow cavities in sedimentary or volcanic rock. These cavities are often created by gas bubbles in lava or the dissolution of organic material like fossils in limestone.

Uses & applications

Primarily for rock collecting, lapidary education, and jewelry/ornamental use once sliced and polished. Also used as interior decor.

Geological facts

Geodes are known as 'nature's lottery' because you don't know the value or beauty of the crystals inside (such as amethyst, quartz, or calcite) until the stone is cracked or sawn open.

Field identification & locations

Identify by a rounded, lumpy, cauliflower-like exterior texture that feels lighter than a solid rock of the same size. Commonly found in deserts, volcanic ash beds, and limestone regions; notable sites include Kentucky, Iowa, and Mexico.

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Sandstone or Siltstone

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

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Sandstone (with man-made markings)

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