Diamonds

Diamonds are a rare ore that can only be mined with an Iron Pickaxe. Diamond ore is the 2nd most rare or in the game; because emeralds are the rarest (can be found in extreme hills biome only)

Layers
Diamonds can be found at the layers 12, 11, or 8. At first, I would go trying for layer 11. (Becareful, you could fall in lava; I recommend shifting!)