what kind of rock do i have?

[tchun]

I am somewhat new to the saltwater "game", but I have been following a friend of mine who has large amounts of coral and fish at his own house. I currently have a 14 gallon biocube and I orignally had 2 big rocks in there. as the tank progressed I decided to try to touch it up for aesthetics. (i.e. break up the rocks to build a better cave to accommodate for my smaller fish, etc.) one rock broke no problem. the other rock did not break after countless efforts, even with a chisel and screwdriver. my friend came over with his industrial hammer to smash the rock, but upon attempting only a small piece the size of my thumb fell off one end. it seems to be a HUGE coral skeleton that must have somehow ended up in a batch of sold rock? my friend and I both were bewildered at the "rock". can someone help me out? maybe first of all tell me what this is in my tank? and second what I can do to maybe break it down? or do i have to find new rock for replacement if i do want to break it down? 

 

attached are some pics. i can post better pics for those that would like it if you can really help me out with my situation. the pic with light on is obviously the rock as two large pieces originally. the picture with the lights off is the one after breaking up the first rock and a flat rock i obtained from a friend. the second rock still remains as its large piece.

[Origami]

The rock does look like an old coral skeleton. Not to worry, though, a lot (if not a majority) of reef rock actually originates from old coral heads.