There are several locations around Arizona where this unique agate can be found. While chalcedony is very common here in the desert, there is a highly sought version of the stuff with 'fire' inside. Three of the states four corners have been known to produce the valuable gemstone.
I have a few pieces, and every once in awhile I put some diamond bits on the rotary tool and try to figure out how to cut it without losing the fire. The fire is beneath the surface but it's difficult to determine how deep it lies. The trick is to cut a nice window to the fire and then carve the piece out of the larger piece. It's a lot like cutting opal, much patience is needed.
More about fire agate.