* Formation: Thunder eggs form within volcanic flows as a result of the rapid cooling of lava. This cooling creates cavities within the flow, which are then filled with silica-rich fluids. Over time, these fluids crystallize, forming the distinctive agate patterns within the thunder egg.

* Composition: They are primarily composed of chalcedony, a microcrystalline form of silica. Other minerals like quartz, amethyst, and jasper can also be present.

While thunder eggs are found within sedimentary rocks, they themselves are not sedimentary rocks. They are formed within a volcanic environment and are a product of the cooling and crystallization of volcanic material.