What is a geode in mineralogy ?

Geode : definition

A geode is a more or less spherical shaped cavity with walls frequently lined with crystals. A geode can have various origins : simple open cracks, gas bubbles whose pressure keeps the walls away (case of amethysts in lava flows), dissolution cavities (limestone rocks)...

Geodes can reach impressive dimensions and harbor large crystals. Among the most famous are the famous geodes lined with amethyst crystals in the basalts of Rio Grande do Sul, Brazil and Uruguay, one of the largest of which was 10 x 5 x 3 meters (Santa Cruz, Uruguay) and supplied 35 tonnes of crystal covered plates.

Many localities have delivered fabulous crystallizations from multi-metric geodes. These cavities indeed allow crystals to develop freely : they can reach impressive dimensions (several meters) and develop automorphic shapes which further add to their quality. No wonder many of the world's best mineralogical specimens come from geodes.