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Geode identification and geology

A roughly spherical, hollow rock cavity lined with inward-growing crystals, typically quartz amethyst, clear quartz, smoky quartz or calcite. Geodes form when mineral-bearing fluids percolate into voids in volcanic or sedimentary rock and precipitate crystals on the cavity walls over thousands to millions of years. The outer shell is typically a harder material such as chalcedony or limestone. Field: mineralogy. Example: The Keokuk region of Iowa, Illinois, and Missouri is famous for producing geodes lined with quartz, calcite, and pyrite crystals within Mississippian-age limestone.. Related terms: Vug, Crystal habit, Agate, Amethyst.

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Sources, scope & corrections

These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.

Editorial owner
GeoMiner
Last updated
2026-03-30T15:19:41.999576+00:00
Review status
Published; not independently peer-reviewed
  • USGS Thesaurus — U.S. Geological Survey. Controlled earth-science terminology used to describe USGS information resources.
  • Glossary of Geologic Terms — U.S. National Park Service. Geologic definitions prepared by the NPS Geologic Resources Inventory.

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Common questions

What does Geode mean in geology?
A roughly spherical, hollow rock cavity lined with inward-growing crystals, typically quartz amethyst, clear quartz, smoky quartz or calcite. Geodes form when mineral-bearing fluids percolate into voids in volcanic or sedimentary rock and precipitate crystals on the cavity walls over thousands to millions of years. The outer shell is typically a harder material such as chalcedony or limestone.
What is an example of Geode?
The Keokuk region of Iowa, Illinois, and Missouri is famous for producing geodes lined with quartz, calcite, and pyrite crystals within Mississippian-age limestone.
Which earth-science field uses the term Geode?
Geode is used in mineralogy.
Which terms are related to Geode?
Related terms in this reference are Vug, Crystal habit, Agate, Amethyst.

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