Specimen is 100g(3.5 oz) and the mineral stand has been included. It has been cut and polished.
It formed around 60 million years ago, as a result of volcanic eruptions in what is now Oregon, and deposited deposited thick layers of rhyolitic lava. This lava was rich in silica and formed porous, gas-filled flows.
 
As the lava cooled, it contracted and cracked, creating hollow spaces and gas pockets inside the flow. These cavities became the birthplace of thundereggs.
 
Over time, groundwater rich in dissolved silica, quartz, and other minerals seeped into the cavities. The exact composition of these minerals depended on the local geology, which is why different thunderegg beds have unique colors and patterns.
 
The dissolved minerals gradually precipitated inside the hollow spaces, forming agate, jasper, opal, and other quartz varieties. Some thundereggs completely filled with agate, while others remained partially hollow with stunning crystal centers.