I remember when I was younger finding some "rocks" that when split open had a hollow space filled with crystals. I never knew why they looked like all the other rocks but were so different on the inside. Since we're in a unit learning about rocks and minerals, I thought it would be interesting to learn more about the geodes.
The only way if you can tell something is a geode is to break it open. On the outside geodes look like regular rocks. Geodes contain minerals which have been dissolved in silica precipitation. These minerals form into crystals when the silica cools. This happens over a long period of time, thousands of years. The most common mineral found in the core of a geode is quartz. Geodes come in many different shapes and sizes and are most commonly found in the desert, although you can find them all around the world.
More on geodes: http://www.desertusa.com/magjan98/jan_pap/du_rock_geode.html
Picture from: http://www.rootsweb.ancestry.com/~iadesmoi/Toomstone/Augusta/geode.jpg
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