Geodes
We picked up some Geodes to break apart at home when we were at the Science Museum last week. Look how beautiful they are.
Geodes are hollow, crystal-lined rock cavities found in sedimentary rocks like limestone (calcium carbonate) and dolomite (calcium magnesium carbonate) or in volcanic rocks. In sedimentary rocks, geodes may form by dissolving out of cavities by ground water and redepositing of minerals as crystals which usually point towards the center of the cavity.
Geodes begin as bubbles in volcanic rock or as animal burrows, tree roots or mud balls in sedimentary rock. Over time, the outer shell of the spherical shape hardens, and water containing silica precipitation forms on the inside walls of the hollow cavity within the geode. The silica precipitation can contain any variety of dissolved minerals, the most common being quartz, but amethyst and calcite are also found.
Over a period of thousands of years, layers of silica cool, forming crystals of different minerals within the cavity. Different types of silica cool at varying temperatures, thus creating layers of different types of mineral crystals.
Geodes are found throughout the world, but the most concentrated areas are located in the deserts. Volcanic ash beds, or regions containing limestone, are common geode locations.
There are many easily accessible geode collecting sites in the western United States, including California, Arizona, Utah and Nevada. The state of Iowa is also has geodes, in fact, the geode is their state rock.
