Geodes are rock formations which occur in sedimentary and certain volcanic rocks. They are essentially rock cavities with internal crystal formations or concentric banding. Their exterior is generally limestone or a related rock and the interior contains Quartz crystals and/or Chalcedony deposits. Other varieties are completely filled with crystal and are solid all the way through and these types are known as 'nodules'.
The name geode originates from the Greek meaning 'earth like'.
Further reading:
Geode
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