What's this Rock called?

that's a geode. here's a link: http://en.wikipedia.org/wiki/Geode here's how you open them: http://www.geodegallery.com/breakinggeodes.html

grahamma,

It sounds like what you have is a geode. To get at the inside, you have to break it open into at least two pieces, but you don't want it to shatter into many pieces so you can enjoy the array of crystals on the inside. I have been collecting things like this for many years, and here's the best way I've found to break it open: Place it into an old sock, or some other cloth bag, take it outside to a hard surface (pavement or sidewalk), and give it a moderate whack with a hammer. After a couple of tries, the geode should break in two.

It's called a geode, different geodes have different minerals in them, like amethyst, citrine, agate ect. I'm not sure how to open it without damaging at lease some of the formation, just carefully with a chisel and hammer I guess.

It is a geode. The advice about a rock club is sound and beats anything you may try to do one your own.

it's probably a geode (gee-ode). best way to break apart would be very carefully with a hammer and chisel.

You can cut it open with a diamond disc cutter, hire that from any tool hire shop or any block paving layer will have one.

It sounds like a "thunder egg"

You can get it cut with a diamond saw at a rockhound club.

goed buy a rock saw