This is an Oregon thunderegg, which formed through a fascinating volcanic process that took place millions of years. The specimen weighs 80 g ( 2.8 oz ), with the stand included.
Around 60 million years ago, volcanic eruptions in what is now Oregon 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. One of these cavities became the birthplace of this thunderegg. Over time, groundwater rich in dissolved silica, quartz, and other minerals seeped into the cavity. 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 space, forming agate and other quartz varieties. This thunderegg is completely filled with agate.