DONT USE DOED!! Ankylo or Phoenix is the best by far, this is from personal experience. Doed gets you only stone from MOST the obsidian around, on some maps there is another kind which has metal too, that is the ONLY kind a doed can even harvest at all.
PHOENIX IS BADASS!! mine was 37 b4 tame, and now has replaced my quetz cause it has a weight over 1200, and it can harvest all the same stuff as an Ankylo, not as good, but not much worse tbh.
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.
In the island. Find it in snow biome or inside cave, u will find lot of it. if u see very black and shining stone, thats an obsidian. Dont need to climb a high mountain or going to far volcano, (well its fine if u had argentav). Just be ware with the wild creatures