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Typical color of the thundereggs	Unusually colorful thundereggs

.    "Thunderegg" is a common name given to the Lithophysae type nodules with geometrically shaped cavities surrounded by rhyolite shells.

There are several ideas on how the thundereggs may have formed. Let's start with the most common one, the "Hot" idea.

    Unlike the gas bubbles being preserved in the solidified lava flows, the gases opened up the cavities within the molten rock "bubbles" floating in the perlite/rhyolite flows (think of lava lamps). The "bubbles" hardened into the balls of radiating needlelike H-Cristobalite, a type of quartz that can form only at higher temperature, and feldspar crystals may be also involved. While the cristobalite "bubbles" were still very soft, and not yet solidified, dissolved gases eventually gathered together, and forced open a cavity between the layers of radiating structures within the "bubble. The fact that the lava was shrinking when cooling may had helped drive the opening of the cavities.  That was how the cavities in the thundereggs are comparable to the way the Africa and South America continents fit each other together, and the radiating structure of the thundereggs was the main reason for the star or crescent shaped cavities in many thundereggs.
    Eventually, the water passed through the fractures within the solidified lava rocks, altering the rocks and leeching off the minerals in the process and some sensitive lava rocks especially perlite, a hydrated form of volcanic glass, decomposed into clay layers. At the same time, the mineral-rich solutions filled in the cavities of the thundereggs, resulting in the deposition of the agate-forming solutions. while "petrifying" the thunderegg shells with harder form of quartz.
     It was said that each band of the banded agate represents each period when water was available kind of like tree growth rings but in reversed manner despite some unexplainable characteristics. The impurities such as iron also were included in silica solutions, and they acted as coloring agents that provided colors to the agates. The interruption of the mineral rich solutions as in case of changing from wetter environment to drier one, can cause the formation of the agates to fail to be completed, hence, the hollow centers of the agate nodules. The shift back to the wetter environment can resume the building up of the agates, and the cavities can be lined or filled with the quartz crystals if the environment favors that. It must be noted that rhyolite and pertile lava rocks tend to be very poor in iron, an important coloring agent for the agates, so it is rare for any agates in any thundereggs to be colored. 
   The best thundereggs were found in the clay beds that had been weathered from the perlite lava beds as well as any slightly weathered perlite lava beds. The perlite beds were associated with rhyolite and obsidian beds as in case of those found in Oregon, Wyoming, Nevada, Washington, Arizona, California and New Mexico of USA.

The other idea ("Cold" Idea) was that the thundereggs were formed like the Septarans in the ash/tuff/decomposed perlite beds. Eventually, the beds either get heated or wet, the nodules would be crystallinzed in radiating manner and when the beds start to either cool or dry, the nodules struggle to shrink and got ruptured from within with support of something like vapors within. The solutions later filled the cavities with quartz, opal and agates.