Others may have any one of a variety of opal fillings that may be opaque blue, opaque red, translucent pastel blue, translucent yellow, translucent red, white, or colorless. Some of the opal can be faceted, and a small percentage is true precious opal. Some eggs have well-developed calcite crystals encased in chalcedony, and others contain pseudomorphs of chalcedony after calcite. Some eggs have layering that is fanned from one edge, because the egg was rotated by earth movement while the filling was being deposited. This and other features suggest that the complete development of some eggs may have taken considerable time, and the filling-in of the egg may have recorded a series of geologic events.
Some eggs contain brecciated rock fragments, while others show faulting, offset, and healing.
One of the most unusual Thunderegg variants is up to 3 feet long and 2 to 3 inches in diameter and looks much like a fat gray worm. In some areas, it is common to find the characteristic chalcedony core weathered out of its shell. If a complete egg is sawed in the right orientation, one or more conduits through which filling materials flowed may be found. The beauty and complexities of many of the cut and polished eggs explain why Oregon rockhounds have long been fascinated by Thundereggs. Some localities occur in beautiful forested hill country, others in dry, desertlike terrain.