Coolest rock I've ever seen

When I was tumbling my mix of rocks, I found this one at the bottom of the bag. I only put it through the week of polish because I love the way it looks and didn't want anything to happen to it in the other grit. I don't know what it is, so if you'd like to suggest something go for it!

EDIT : I asked my buddy Tim, who rocks (pun intended, he works at a very cool local rock shop and is very knowledgeable.) He said it looks like chalcedony, but that's all he told me.
But still, the closest picture I could find to this is [link]