Thunder Eggs, aka Lithophysae, are a type of igneous
mineral formation that look like rocks from the outside.
The outer shell is made of rhyolite, a type of volcanic rock made of quartz and feldspar. As lava cools, it releases steam and gases, which can cause bubbles. Sometimes the lava cools around the bubbles, forming hollow rocks.
The inside of thunder eggs is formed when water carrying silica seeps in, leaving mineral deposits. The rocks become filled with agate, chalcedony, jasper, amethyst, opal, quartz crystal, or a combination of these.
Although plain-looking on the outside, thunder eggs are very beautiful when they are cut open and polished, revealing the patterns inside. The mineral deposits can form stripes, stars, flowers, or just random designs.
The name comes from a Warm Springs Indian legend about a fight between the Thunder Spirits of Mount Hood and Mount Jefferson. The Spirits threw "rocks" at each other - which had been stolen from the nest of the Thunderbird.
The thunder egg is the official State Rock of Oregon.