Tutorials/Diamonds

This explains how to find Diamonds in Minecraft. You should have at least Iron Tools by this point.

By Cave
Caving is the easiest way to find a Diamond but this method exposes you to mobs including creepers. However, you need to find a cave which goes down to the lava layer (layer 10), if you haven't already found one. Look in your general area for any caves, and explore any caves fully to see if they lead down to lava. Chances are, if the caves branch, it is a very big cave, and most likely will go down to lava. Explore around the lava layer until you find a Diamond Ore. Although it may seem easy to find the Diamond Ore, most of the time parts of the vein will have been destroyed by the cave generation so Mining is a more efficient, if time-consuming technique.

By Mining
Dig down to layer 14 or lower (if you see a lava source block, you're there.) If you find a cave while digging down to this layer, explore it, and use the above paragraph.

Once you hit the layer, dig in one direction. Dig "branches" of this hallway every two blocks, ensuring the biggest area covered. Again, if you find a cave while digging your cave, explore it for Diamonds. Repeat this until you find a Diamond Ore.

Mining a Diamond, and Getting It Back Safely
Once you find a Diamond Ore, dig around it to uncover any other ore blocks. Diamond veins can connect by corners, and you don't want to miss a single Diamond!

Now mine the diamond ores, with an iron or diamond pick. For maximum yield, use a pickaxe enchanted with Fortune. However, the Fortune enchantment is relatively difficult to obtain while enchanting. If you do manage to get a Fortune enchanted pick, the net profits in diamonds can be high.

When in a cave, a simple way of safely getting a diamond is to dig a hole next to the diamond, putting water in that hole, and then mining the diamond. This will mitigate the chance of losing your diamond to lava.

If you dug around to get your Diamond, the task of getting it back safely is easy. For a cave, it's much harder. If you don't know where the entrance to your cave is, you can always dig a stairway up until you hit the surface, and then find your house from there.

In A Cave
Explore more. Enough said.

In A Strip Mine
Diamonds will spawn below layer 16, so you can do a staggered Branch Mine, with tunnels at layer 14 (covering 14, 15, 16), 11 (covering 11, 12, 13), and 8 (covering 8, 9, 10).

It would look something like this: MCMMCMMCMMCMMCM C.CC.CC.CC.CC.C C.CC.CC.CC.CC.C MCMMCMMCMMCMMCM MCMMCMMCMMCMMCM C.CC.CC.CC.CC.C C.CC.CC.CC.CC.C MCMMCMMCMMCMMCM C - Covered Area . - Tunnel M - Missed Area

Another way to mine is to mine like this: MMCMMMCMMMCMMMCM MC.CMC.CMC.CMC.C MC.CMC.CMC.CMC.C CCCCCCCCCCCCCCCC .CMC.CMC.CMC.CMC .CMC.CMC.CMC.CMC CMMMCMMMCMMMCMMM C - Covered Area . - Tunnel M - Missed Area This branch mining works because most diamond deposits are at least 2 wide or 2 tall. It leaves little missed area with the most coverage.

In other mines
There are other ways to mine diamond, but they may or may not be so efficient. Here is a list to other tutorials for mines
 * Tutorials/Quarry
 * Tutorials/Branch Mining
 * Tutorials/Vertical Mine Shaft With Water Drop