What tool do i use to break diamond in minecraft?

Iron pickaxe

I'm glad you asked! Minecraft can be confusing if you haven't played it a long time. You need to use an iron pick axe. You can get an iron pick axe by mining iron with a stone pick axe and putting it in your oven to make an iron ore. Then, make sure you have some sticks. With your crafting table, you can make an iron pick axe and start mining away!

Iron pickaxe

Iron pickaxe
here is a little chart to help you out.
wood breaks stone, coal
stone breaks stone, coal, iron, and lapus lazuli
iron breaks stone, coal, gold, lapus lazuli, diamond, and redstone
gold breaks stone and coal (gold is very noneffective in the use of tools. It breaks very quickly. The only advantage is that it harvests blocks fast)
diamond breaks obsidian and all other blocks mentioned above