Geodes (Before)
(c) 2002 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Geodes are mysterious lumps that occur in certain limestones. These two, shown almost life size, are typical examples. They can be as large as about 30 centimeters across. If you look at these geodes closely, they look as if they started out smaller; something grew inside them and forced their sides apart. Geodes aren't made of limestone, but rather the much harder chalcedony. That's the hydrous silica mineral that makes up chert.
Many commercial "geode mines" in limestone country give you the chance to harvest your own geodes from the weathered bedrock. I didn't dig these up but instead paid a few bucks for them at a rock shop.
Why? Because geodes might have treasure inside. Next . . . >
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society
