Geode (Chert or Quartz Nodule)

Physical Properties

Hardness: 7 (Mohs scale); Color: Off-white, tan, or grey exterior; Luster: Dull/earthy exterior, vitreous (glassy) interior; Structure: Microcrystalline or cryptocrystalline; Specific Gravity: Approx 2.65.

Formation & Geological History

Formed through the chemical precipitation of silica-rich groundwater within cavities of sedimentary or volcanic host rock. Most sedimentary geodes form in limestone or shale over millions of years.

Uses & Applications

Primarily used for lapidary work, home decor, educational collecting, and spiritual purposes.

Geological Facts

Geodes are often referred to as 'thunder eggs' when formed in volcanic ash. While they look like ordinary rocks on the outside, they can contain hollow centers lined with sparkling crystals or colorful agate banding.

Field Identification & Locations

Identify in the field by their rounded or 'cauliflower' bumpy texture and lighter weight compared to solid rocks. They are common in the American Midwest (Keokuk region) and Southwest, as well as Brazil and Uruguay.

Identified on: 4/24/2026

Mode: Standard