A Thunderegg rock is a nodule like structure, much similar to the Geode rock in appearance. It is evolved inside the layer of rhyolitic volcanic ash. It can be found in different sizes varying from an inch to a meter, but generally it is found in a spherical shape of about a baseball size. The center of the structure basically contains chalcedony, composed of agate, opal or jasper, sometimes all, uniquely or in a combination. The other crystals found are gypsum and quartz.
Thunderegg has the ordinary look of a rock from the exterior part, but the beautiful colors and intricate patterns are visible only on slicing it in half. Like the other agates, the thundereggs acquired from the same location of the bed may differ in color and appearance, but they share a specific identity between them. This geological structure is found commonly in the locations of Oregano, USA. The other places like Germany, Australia, Mexico, Poland, Canada, Argentina, Turkey, France and Romania have a good collection of Thunderegg rocks.
Thundereggs are developed from the Rhyolite lava. It is believed to have been formed in the lava’s gas pockets, which work as moulds. The water percolates through the rocks and cools the bubbles formed in the lava pockets. The bubbles are gradually filled with silica mixed water. This solution deposits in the cavity and the surrounding areas along with a dark shade matrix material. The inner part gets loaded with chalcedony and agate. The different colors in the Thunderegg are obtained from the minerals present in the soil.