There are two ways to get diamonds in minecraft.
1.) Mine them.
Diamond ore can be found anywhere below ground, and only in the first 16 layers above bedrock. They can only be mined with an iron or diamond pickaxe. A good diamond mining strategy is called "branch mining,"
2.) Use creative mode or a mod to get them.
If you checked "enable cheats" when creating the world, you can switch to creative mode, get as many diamonds as you want, and then switch back to survival. To do this, open up the chat. (deafult key T) Then, type "/gamemode 1" and press enter. Open the inventory, put as many diamonds as you want in your inventory, and then open chat again and type, "/gamemode 0" to switch back to survival. IF you want to use a mod, download and install the "too many items" mod.
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You can find diamond ore 16 above bedrock to bedrock then you use an iron pick to mine it. you can also find diamond picks in abondoned mine shafts added in 1.8.
Diamond is 10th layer and below
There is a Minecraft song called 'In Search of Diamonds'.
To get diamond, you need a pickaxe of iron, gold or diamond.
Diamond Armor, Diamond Tools, other stuff like Jukeboxes. The rarest material of all.. diamonds. You find some, you should be happy Diamond is the highest tier in UNMODDED minecraft. See DIAMOND.
You can make a diamond sword in Minecraft by placing two diamonds above a stick in the crafting table.