Are nodule-like geological structures, similar to a geodes, that are formed within some lava flows.
They can range from less than an inch to over a foot across. They usually contain center of agate, jasper or opal. Also often, they have quartz or selenite crystals, and other mineral growths. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colors.
Thunderegg is not synonymous with either geode or agate. A geode is the name for a rock with a hollow in it, often with crystal growth. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form.
According to Pacific Northwest native legend, the Thunder Spirits lived at the highest reaches of Mount Hood and Mount Jefferson, and when they became angry at each other, they threw these spherical rocks at each other.
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