geode

A geode is a hollow rock cavity lined with mineral crystals. Geodes are typically spherical or oblong and range from a few centimeters to several decimeters in diameter. The exterior rind is usually a dense layer of chalcedony or other silica-rich material, which protects the hollow interior. Inside, crystals grow from mineral-rich fluids that seep into the cavity after the rock forms.

Geodes form in two main settings: volcanic and sedimentary. In volcanic rocks, gas bubbles become cavities that later fill with mineral-rich solutions. In sedimentary environments, nodules or pockets in limestone or similar rocks create voids later coated with crystals.

Common minerals filling geodes include quartz (often clear, smoky, or purple as amethyst), calcite, celestine, and gypsum. Quartz geodes are among the most common; agate or drusy formations may cover the interior. Crystal size and color depend on chemical impurities and formation conditions. Geodes can take thousands to millions of years to mature.

Geodes occur worldwide. Notable producing regions include Brazil and Uruguay (amethyst geodes); Mexico; and several areas in the United States, such as Iowa (Keokuk geodes), Kentucky, Ohio, and Arizona. Other important sources include Madagascar, Serbia, Romania, and Morocco.

Uses and significance: Geodes are sought as decorative specimens and in lapidary work; they are popular in collections and educational settings for illustrating secondary mineralization and cavity growth in rocks.