Corundum is a new metal/element added in the most recent update. It is the most strongest metal in-game at the moment, and is among the most common to find, if you know where to look.
In the first test server, when you mined Corundum, you would get 3-4 block of it like you would Redstone Dust. They quickly changed it, so now you only get 1 block.
In the test server, Corundum could only be mined with a Diamond Pickaxe. It can now be mined with Iron Pickaxe strength and above.
Corundum blocks can be found from 5 (first layer of bedrock) up to 16. It's most commonly found in the range of 10-16.
Corundum blocks can be smelted in a furnace to make Corundum Ingot's.
See Corundum Ore for more information.