Thundereggs are nodular rhyolitic formations which can have hollow cavities and can contain a star, triangle, or lens shaped interior. These can fill with any type of agate, jasper, or opal, or any other mineral. In cases where they are hollow, they are usually referred to as geodes (Dugway Geodes, for example, are actually thunderegg formations). Thundereggs got there name from the tale that the gods on Mount Hood were warring with the gods on Mount Jefferson. Since neither had any weapons, they stole eggs from the Great Thunderbird. As the war waged on, these eggs fell to earth in clusters and formed the thunderegg deposits we know today.
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