Why can't I mine this block?

I've hammered away at the ore block in this screenshot (which I believe is diamond) but I can't break it. I've tried other tools but none have any effect. Can anyone suggest what I'm doing wrong?

This is not diamond ore. This is bedrock, a material that is only naturally found at the very bottom of a world (lowest 2 or 3 layers) in both the Overworld and the Nether, or at the top of the Nether. It serves as a barrier between you and the certain-death void layer. You cannot mine it and you can only destroy it in Creative mode. Even if you use creative mode to spawn a block in, it still has no uses in any recipe whatsoever.

For reference, Diamond ore looks like this:

If you want to find diamond ore, your best bet is to go up 2 or 3 levels, then start a strip mine, as explained in What's the most efficient Minecraft mining strategy?

That is bedrock, Unbreakable blocks (Well, in vanilla atleast), that can withstand everything. Without these you'd be falling in the Void now, and die. This is not diamond ore. Diamond ore is found around Y: 1 to 15, and appear mostly in groups of 3 to 5, and Diamond ore looks basically like coal or iron, but instead being black or "Brown?", Diamond Ore has a bright blue color. (The Darker blue ones are Lapis Lazuli, which is a dye) Diamond ore requires an Iron pickaxe or Higher, such as a Diamond pickaxe. If you try to break it with Stone, wood or gold it will just break with no yield. Diamond doesn't come in ore (Except if SilkTouch is used), but drops like the gem itself. Like coal, for example. If you're mining diamond, and you have a Fortune pickaxe, make sure to use it, as you'll be getting more diamonds from one ore.