Thunderegg cores of which the shell has fallen off due to weathering are fun to look at.
Structures visible at the outside of the core are the negative prints of the cavity in thunderegg; sometimes casts of crystals that once were present in the cavity, can be seen too. On many cores signs of spheruletic growth of cristobalite crystals in form of rooftile patterns can be found.
Besides giving clues of the forming history, looking at forms of cores may help making an imagination how the shape of a core in a whole thunderegg could look like. It helps judging how to cut the egg to get a nice thunderegg face. On this page 4 different types of cores.
Biconoid shaped cores. The first thunderegg above is from the NJ01 bed. The shell is intact. There is only one sharp outcropping ridge visible, a ‘pressure ridge’. The other video shows a thunderegg core (Mirador/Argentina), with a shape as can be expected in the intact thunderegg at the left. The shape is called ‘biconoid’; a cut core has two sharply formed edges. It is the most simple form of a cavity/core.
Triconoid shaped cores. A cut triconoid core has three sharply formed edges, an extra fissure opened up compared to the biconoid cores. First video is showing a core, still with some shell attached. Second has a intact shell, with pressure ridges which can be followed around the egg. Both examples from Eddy bed.
Boxcores. The thundereggs above are from the Richardsons Ranch in the USA. First one shows a completly weathered core, it is called a boxcore because of the box shape of it. A cut boxcore has four more or less blunt edges. The second thunderegg only has lost its cap. The topside of the core is visible and also the sharp ridges of the core are peaking through the eggs shell.