Geode (unopened) or Calcareous Nodule

Physical Properties

Rough, bumpy exterior, dull luster. Often somewhat spherical or knobby. Hardness varies (3 for calcite, 7 for quartz/chalcedony exterior). Color is typically light gray, tan, or chalky white.

Formation & Geological History

Formed in sedimentary rocks like limestone or shale. Minerals precipitate from groundwater into cavities or around a nucleus over millions of years.

Uses & Applications

Often collected to be broken or sawed open to reveal potential crystal formations inside. Also sold as novelty items.

Geological Facts

Geodes look like ordinary, ugly rocks on the outside but can contain beautiful crystals (like amethyst or clear quartz) on the inside.

Field Identification & Locations

Identify by its spherical or knobby shape, rough exterior somewhat cauliflower-like, and feeling relatively lighter than a solid rock if hollow. Found in sedimentary deposits worldwide, notably in the midwestern US.

Identified on: 4/26/2026

Mode: Advanced