Geodes are not found randomly here and there. Instead they are usually found in large numbers in areas where the rocks have formed in a special geochemical environment. Most geodes localities are in A) stratified volcanic deposits such as basalts and tuffs; or B) stratified sedimentary carbonate deposits such as limestones and dolomites. A diversity of other environments yield a small number of geodes.

Geodes form in many different ways, and there are a number of valid theories about their formation. The intent of this article is not to provide a single or a comprehensive coverage of the many ways that geodes might form.