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 "Amygdaloid" refers to the gas bubbles trapped in the solidified basaltic lava rocks that would be later filled with minerals. The lava flows spawned out of the volcanos, and the gas bubbles began to appear within the lava flows. The gas bubbles would attempt to escape from the lava flows, and some gas bubbles became trapped when the lava solidified, forming the rounded or dome shaped cavities. Eventually, the solidified lava bed got buried by more lava or sediments. Later, mineral rich water passed through the fractures within the lava rocks connecting to the cavties where some minerals were deposited as in case of silica solutions deposited in the cavities 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 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. The amygdaloid agates are generally shaped like pitted teardrops, rotten potatos, and dome shaped with flat bottoms, and they are found only in the dark basalt and andesite lava beds especially around the fissures or those with ready access to the source of solutions. 
This agate lost a chance to complete itself, and ended up as a hollow geode. More information about how the agates and thundereggs formed can be
found in this section:
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