Geodes are natural inorganic objects which are hollow. Geodes are usually roughly spherical in shape and can occur in igneous or sedimentary rock. The interior may be lined with crystals, usually quartz, pointing toward the center. Quartz consists esentially of the elements silicon and oxygen, but exhibits many varieties of color and form. Geodes are sometimes lined with chalcedony, a variety of quartz and, on occasion, closely related opal.
The hollow interior of the geode is it's most characteristic feature. If the interior of a formation is completely filled in, it is then classified as a nodule, not a geode. Nodules can be composed of a number of minerals, such as agate, quartz or calcite.
Geodes usually occupy the sites of former gas cavities in the volcanic rocks, basalt, rhyollite and tuff. Geodes can also occur in various other environments, such as sedimentary rock, which occurs as shale formed by ancient oceans. The interiors of geodes are very different from their exteriors and until a geode is opened, there is no easy way to determine what is inside.
If you have additional information about geodes, unique or interesting ways to open them, or information on geode collecting sites, please leave a comment to this post.