sedimentary

Quartz Geode

Silicon Dioxide (SiO2) within a secondary sedimentary structure

Hardness: 7 (Mohs scale); Color: Clear to milky white crystals with a tan/brown exterior rind; Luster: Vitreous (glassy) inner crystals, dull/earthy exterior; Structure: Hexagonal crystal system; Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Clear to milky white crystals with a tan/brown exterior rind
Luster: Vitreous (glassy) inner crystals, dull/earthy exterior

Identified March 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in cavities within sedimentary rocks (typically limestone or shale) where mineral-rich groundwater deposits silica over thousands of years. Often dating from the Mississippian or Ordovician periods.

Uses & applications

Primarily used as decorative display pieces, educational geological specimens, and in metaphysical/retail markets. Smaller pieces are sometimes cut for jewelry components.

Geological facts

Geodes are essentially 'hollow rocks' that look like ordinary stones on the outside. The Keokuk geode from Iowa is perhaps the most famous variety and is the state rock of Iowa.

Field identification & locations

Identify in the field by looking for rounded, lumpy nodules that feel unusually light for their size. Common in areas like the 'Geode State Park' in Iowa, Morocco, and various desert regions in the US Southwest.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

Arenite (primarily SiO2)

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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)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary