sedimentary

Quartz Geode

Crystalline Silicon Dioxide (SiO2) within a nodule

Hardness: 7 (Mohs scale); Color: Off-white/tan exterior, typically clear or white crystals inside; Luster: Dull/earthy exterior, vitreous (glassy) interior; Crystal Structure: Hexagonal; Cleavage: None; Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Luster: Dull/earthy exterior, vitreous (glassy) interior

Identified May 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in cavities within sedimentary rocks (like limestone) or volcanic rocks. Mineral-rich groundwater seeps into hollow spaces, depositing layers of minerals over millions of years. Common in Mississippian-age strata.

Uses & applications

Primarily used for geological education, home decor, lapidary art, and mineral collecting. Large, high-quality geodes are popular display pieces.

Geological facts

Geodes are often nicknamed 'thunder eggs' in some regions. They are distinguished from nodules because they contain a hollow cavity or crystal linings. The word 'geode' comes from the Greek word 'Geoides' meaning earth-like.

Field identification & locations

Identify by the rounded, cauliflower-like exterior texture and lighter-than-expected weight if hollow. Commonly found in desert regions, stream beds, or limestone outcrops. Use a rock hammer to carefully crack or a saw to cut open.

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Sandstone or Siltstone

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Sandstone

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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)

Arenite (SiO2)

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Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

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