Photo by Pat McMahan from http://www.agateswithinclusions.com/images/US/ore/Priday_Ranch_Plume6.jpg

The thunder egg was declared the official state rock of Oregon in 1965, because there are quite a lot of them there. A thunder egg is a rounded nodule or geode with agate in the center.  Thunder eggs can also contain quartz, chalcedony, crystals, or opal.  The inside parts can be opaque or transparent – there are almost as many possible designs as there are agates.  This unusual thunder egg shown above is from Oregon and has plume inclusions. Some thunder eggs are also geodes but this one is not a geode because it does not have crystal points. A more typical thunder egg would look similar, but with bands or a single color on the inner part.