How do geodes form?

We put this question to Ian Mercer:

Well, we were talking about volcanoes earlier. Now, if you think of a volcano after it erupted and all this black lava comes out and it starts cooling down then, as it cools, some of the gases start fizzing out. They form big bubbles and those bubbles gradually fill with more minerals as the water flows it cools the lava. These minerals line the side of what was the bubble and because the lava very often turns to a sort of soil and wears away you're left with this hard mass which was the lining of the bubble. Some of those when they're broken open you find that they're lined with these beautiful crystals, quite often amethyst which is purple quartz.