I'm not really sure...I'm thinking just Florida rock. I do have about 70% Tonga and its fairly hollow and has lots of holes and such. This type shown (Florida?), I only have a few pieces of and I don't really like it. It's fairly dense and heavy, without hiding holes. Good for a solid base I suppose. I happened to get this one though, just because of the markings shown in the pic. I can't figure if it's a coral type skeleton or what, but it really looks cool in real life. Very alien like! (The pic does not do it justice). Maybe something will start seeding from it, I dunno... There is some nice coraline on it too and the tree looking algea on it.
Billb