[cogitator]

What is the difference between a "thunder egg" and a "geode"?

Two things: geodes are partially hollow inside, allowing the formation of crystals, rather than the more common contents of thunder eggs, various forms of amorphous silica (agate, jasper), including some with valuable opal.

Second, and perhaps to geologists more importantly, geodes form in different environments; thunder eggs always occur in volcanic rhyolites. Geodes can occur in igneous or sedimentary formations -- in sedimentary formations the outer shell is limestone or dolomite.

I knew some of this (especially the distinction between partially hollow and solid-filled), but I learned more here:

Concretions, thunder eggs, and geodes

[DocRock]

I knew an old geologist that used the term Vugh when referring to geodes.

[Jemian]

Thanks. I have learned something today. Good on ya'!