Geode (Unopened)

Physical Properties

Hardness: 7 (Mohs scale for outer shell); Color: Tan, beige, or brownish outer rind; Luster: Dull/earthy exterior; Crystal structure: Trigonal (often containing internal quartz/chalcedony); Cleavage: None (conchoidal fracture when broken).

Formation & Geological History

Formed in voids created by bubbles in volcanic rock or hollows in sedimentary rock (limestone). Over time, mineral-rich groundwater seeps in, depositing microcrystalline quartz (chalcedony) and macrocrystalline quartz layers.

Uses & Applications

Primary use as collector specimens, lapidary education, decorative display, and jewelry (once cut and polished to reveal the crystal core).

Geological Facts

Geodes are often referred to as 'thunder eggs' in certain regions. The word geode comes from the Greek word 'Geoides' meaning 'earth-like'. They are Missouri's and Iowa's official state rocks.

Field Identification & Locations

Identify by their rounded, lumpy, or cauliflower-like exterior and unusually light weight for their size (suggesting a hollow center). Common locations include the Keokuk region of Iowa, Illinois, and Missouri.

Identified on: 4/23/2026

Mode: Standard