Meribeth, you asked a very good question

I'm going to do my best to answer Meribeth's comment on the thunder egg post..

And, I've never seen a meteorite geode but will look them up asap. They sound so interesting! They must have formed when the rock matrix entered the upper atmosphere.

Meribeth, the short theory is that they are formed by hydro thermal- or even by cooler solutions of silica charged water - traveling into pockets, the silicas being leached out of the matrix.

Some geodes have levels of criss-crossing banded agate layers inside, which seems to show a process over time. Geodes with nice quartz crystals inside usually form in ash layers and sometimes get washed away afterwards into other places, enmasse pile-ups if you will, in ancient floods,etc. The Blue Grotto in Italy is a big Geode.

At Quartzsite and Tucson rock shows there are people from Brazil who sell geode like forms called "vugs" that are five feet tall and are not round but shaped like a giant butternut squash, for want of a better example. Practically anything bigger than that is too expensive for them to ship to the show.

The way to tell the relative emptiness of the inside of a geode at the Fairs or rock shows, is to heft a couple of them, pick the lightest, discard the heaviest one. Just keep going this way and make the guy crazy, but pick the lightest one you can find. Then ask him to cut that one. :o)