If you have all the diamonds you should also have all the obsidian. After a while that thing becomes a dime a dozen.
@The Flame War, eh. I get tired of mining it fairly quickly. If I ever need obsidian, I generally just cast it where it needs to be. Ofc, that doesn't help with crafting recipes that require it, sadly.
@TheStarHawk, I have very stict strip mining rules so I end up with stacks of the stuff.