Coming from one of many thunderegg locations around Germany, the thundereggs of Et. Egidien and Hohenstein look similar that they may have been found in the similar beds that is close to each other.  Considering the fact that their rhyolite shells are coarse-grained, I can see how anyone are confused by whether those thundereggs came from coarse grained pertile lava flows or "welded tuff" even I think that they might be found in the pertile beds judging from the way the grains are oriented along the lava flow as well as opening of the cavities within the thundereggs.  The agates in Et. Egidien are typically bright red sometimes in color combo with orange, purple, yellow, brown and white. The funny thing about larger specimens are that their cavities tend to be more bioconoid ("eye/diamond") shaped as if the gases were struggling to expand against the overrall weight of the thunderegg itself .  Those from Hohenstein are similar in many respects except that the color of the thunderegg shells tend to be more consistent with more orangish to yellowish agates. Apparently the site is closed, but it had been heavily mined in the past resulting in one of most widely available thundereggs coming from Germany.