.

Typical color of the thundereggs	Unusually colorful thundereggs

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

    Unlike the gas bubbles being preserved in the solidified lava flows, the gases just open up the cavities within the molten rock "bubbles" floating in the pertile/rhyolite flows (think of lava lamps). The "bubbles" wouild harden 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" are still very soft, and not yet solidified, the dissolved gases would eventually gather together, and force open a cavity between the layers of radiating structures within the "bubble. That is why 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 is the main reason for the star or crescent shaped cavities in many thundereggs.
    Eventually, the water would pass through the fractures within the lava rocks, altering the rocks and leeching off the minerals in the process and that's how some sensitive lava rocks especially pertile decompose into clay layers. At the same time, the minera-rich water fill 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 is 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 act as coloring agents that provide colors to the agates. The interruption of the mineral rich water supply 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 cavities in the center of the agate nodules. The shift back to the wetter enivorment 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 in iron, an important coloring agent for the agates, so it is rare for any agates in any thundereggs to be well colored. 
   The best thundereggs are found in the clay beds that have been weathered from the pertile lava beds as well as any slightly weathered pertile lava beds. The pertile beds are associated with rhyolite and obsidian beds as in case of those found in Oregon and New Mexico of USA.