It is a structure, sometimes a nodule, sometimes a geode, occurring in rhyolite, welded tuff, or perlitic rocks. Scientists do not agree on the processes forming Thundereggs. Some insist that the characteristic and unique internal pattern of typical Thundereggs is due to expansion and rupture of rock by gases. Whatever the process, after the cavity that contains the egg is formed, further development is extremely variable in the amount of time needed to complete the egg, degree and type of infilling, and physical characteristics. Thundereggs range in size and weight from less than an inch and under one ounce to over a yard in diameter and over a ton in weight.
Most eggs collected are between two and six inches in diameter.
Typically, an egg has a russet-colored outer shell that is often knobby and often has a characteristic ribbed pattern. Frequently, the inside of the outer shell has a relatively thin intermediate or transitional lining. This is sometimes composed of an iron or manganese compound, often with a thin coating of opal or chalcedony. Sometimes only opal or chalcedony is apparent. Finally, the center of an egg is usually filled with chalcedony or opal and may or may not have inclusions, pattern growth, or crystals.