There are two type of obsidian rocks. Search the one who also drop metal. That's the kind that you should break if you're using a doedicurus. If you break the type that doesn't also drop metal the doedicurus will only harvest stone.
I have 8 of these. i bred a level 343 male and a level 175 female and my baby can now go toe to toe with 10 rexes or 5 lvl 110 gigas. 100% recommend these. also turn off friendly fire so when thay rage they dont kill you.