This is a geode:
You probably have a few. Maybe you’ve even spent a couple bucks at a national park on one you could break open yourself.
What are they?
They’re unassuming rocks with quartz crystals growing in the inside. Usually, like the one in the picture above. they’re amethyst (purple quartz).
How the hey do they form?
I had no idea, so my natural inquisitiveness demanded that I find out. Using my finely-honed research skills (i.e., typing in “How do geodes form” on Google) I discovered something interesting: no one really knows.
However, according to eHow.com (and some other sources) there are two different kinds of geode-creating environments, and a few theories on how they form there.
The first one is my favorite. It says geodes form in –you guessed it– volcanic rock, which is how this topic made it into this blog (just if you were searching for a connection).
Gas bubbles in lava create round cavities. The rock hardens around it, creating a lovely hollow rock. As mineral-laden water seeps into the center, it precipitates the silicate necessary for quartz crystals to grow and voila! Geodes.
The other process is a whole lot more convoluted and non-interesting, but if you want it, follow the eHow.com link above. Something about dolomite and anhydrites.