What are geodes?
Geodes are round rocks containing a cavity that is lined with rock crystals. A casual observer may pass by these rocks with no idea of the treasure contained inside, but if you pay attention and know what to look for, you might be rewarded with a fascinating discovery. Geodes are most often found in desert regions, but can be found almost anywhere including creek beds. I began my interest in geodes when I was in my teens, led on a hike through the woods in Indiana on a geode hunt. I doubted we would find anything, but to my delight, I did bring home several rocks that turned out to be geodes. The insides were nothing special, just a crust of quartz crystals, but it was a thrill to find something so out of the ordinary.
Geodes are partially filled gas cavities from a lava flow, commonly filled with silica (SiO2) in a form of chalcedony. When the cavity fills completely it is called agate. The lava flow that produced the geode is an igneous event but the infilling of minerals (silica, calcite, quartz, etc.) is a metasomatic event (changes in mineralogy due to water under a wide range of pressure/temperature relationships). Metasomatism is a type of metamorphism, which makes geodes metamorphic rocks.
