Geodes (Greek γεώδης - ge-ōdēs, "earthlike") are geological secondary structures which occur in certain sedimentary and volcanic rocks. They are themselves of sedimentary origin formed by chemical precipitation. Geodes are essentially hollow, loosely spheroid to oblate masses of mineral matter that may have formed.
Greenstone -- (Michigan's state gem) was formed in gas bubbles within basalt. Basalt is a volcanic rock, formed in Michigan in part during the Midcontinent Rift approximately one billion years ago. It is found more in the western Upper Peninsula, MI, USA
Obsidian (Volcanic Glass) - Effective grounding stone. Can draw out and release hidden imbalances. Ideal when a transformation is needed, bringing fiery energy from deep within the earth to purify and cleanse.