Lithophysa
They vary in size from very small up to twelve feet in diameter depending on the age of the magma chamber.
These cavities are believed to be caused by expanding gases in tuffs and rhyolitic lavas before solidification.
[1] If the cavity becomes lined with crystals it may be referred to as a geode or if filled partially or fully with agate, jasper or opal it is called a thunderegg.
composed by crystals of quartz,[1] chalcedony,[3] hematite, fluorite[4] and various colored oxides or other minerals.
Lithophysae are also related to the spherulites found in obsidians on the Italian island of Lipari, in Yellowstone National Park, and other places.
1, 2. Displaying typical plates and "sutures".
3, 4. A cluster with three spheroid centers.
5, 6. Example of slow fill structure layers with a large opal layer at the bottom. Expansion is controlled by where the water was located in the structure.
7. "Dimple" and "pimple" where expansion began on an inner shell.
8. Fortification agate