This achievement is gained by finding a diamond ore block and mining it or by picking up a diamond that has already been mined (e.g. if a friend throws you one). This must be done in survival mode. It can be completed on any difficulty.
Unlike on the console versions there is not a tutorial world to easily obtain diamonds from. Hence you need to do this by one of two methods:Method 1:
This is a relatively longer method, but more widely available to people, and is the normal way to acquire diamonds in a survival world.
Firstly, you need an iron pickaxe. Iron is found as an ore which is a brown colour. Craft it as such:
Note: diamonds are usually (but not exclusively) found nearby to lava. Caution is advised when mining diamonds; mine around the ore to make sure it isn't next to lava, otherwise you risk losing the diamonds permanently.
Secondly, you need to find some diamonds. A fast and consistent way to do this is to dig a staircase down until you reach bedrock, then count up 10 blocks going back up from bedrock. From this point mine in a straight 1x2 strip. It shouldn't take long before you happen upon some diamond ore or a lava cave. When making the staircase you may find a cave system, this can often lead down to lava areas, and thus, potentially, diamond ore.
Diamond ore looks like this:
Thirdly, mine the diamond ore with your iron pickaxe, and pick up the diamond. This should net you the achievement.Method 2:
Currently, only local multiplayer is supported; that means that you can play with other people on the same network. So, if you have a sibling or friend on the same network who has already mined some diamonds, then you can pick it up off them or out of their chest. If you take it out of a chest then drop it and pick it up for the achievement. This could also be a good opportunity to get the "Diamonds to You!" achievement.