Dwarves' Earth Treasure:
What is "Thunderegg" and How did they form?

    "Thunderegg" is a common name given to the Lithophysae type nodules with irregular shaped cavities.

    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" are commonly made of the radiating structure (like this symbol * ) of H-Cristobalite, a type of quartz that can form only at higher temperature, and feldspar crystals may be also involved. While the "bubbles" are still very soft, and not yet solidified, the dissolved gases would eventually gather together, and force open a cavity within  each "bubble" by following the weaknesses in the radiating structures. That is why the cavities in the thundereggs are in way the Africa and South America continents can be fitted together, and the radiating structure is the main reason why "star-like" & "triangal" cavities are common in the thundereggs.
    Later, mineral rich water passed through the lava rocks, and some of the minerals were deposited in the gas bubbles as in case of silica solutions being deposited in the bubbles that would eventually form the agates. It is said that each band of the banded agate represents each period when water was available kind of like tree growth rings. The impurities such as iron also get deposited with silica solutions, and they 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 cavities can be lined or filled with the quartz crystals if the environment favors that.
    Note is that rhyolite and pertile lava rocks are always low in iron, an important coloring agent for the agates , thus, it is rare for any agates in any thundereggs to be colored. The thundereggs at the Baker Egg Mine contain most colorful agates found in any thundereggs over the world, and that is due to the fact that the mine's proximity to the basaltic lava rocks which are much higher in iron.

Common color of the thundereggs

Very colorful Baker Egg thundereggs

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