Sugar Geode 6-7"

Geodes can occur within either volcanic or sedimentary rocks. Beginning as hollow bubbles inside other rocks, geodes form over many years. Basically, geodes are sedimentary rocks, the crystals within the bubbles forming as a result of a chemical reaction that leads to precipitation of minerals that become those crystals.

The outer shell of a geode is made of a very hard rock, and crystals form inside geodes only when the perfect combination of temperature, pressure changes and evaporation exists. As water seeps into the rocks around a geode, minerals are deposited inside the hollow rock. Typically, these minerals become agate and quartz, which form in layers very slowly, over the course of thousands of years.