Have you ever been bothered that when you use a diamond pickaxe to mine stone, it uses the same amount of durability when you mine obsidian? I know I have. Since it takes a lot more hits to mine obsidian over stone (I don't know the exact number) The pickaxe should take more of a durability damage right? I don't know exactly how this would work but it feels wrong that it takes the same durability to mine stone than something like diamond or obsidian.
Mining Durability