EDIT: It seems that the coordinates in this solution and comment section no longer work since a recent update and have been changed, will try to do my research and give you the new location (if it exists).
You can find a chest with a diamond in the Sand Temple (X: -229, Y:66, Z:41) in the Tutorial World. Head west and you'll spot the temple near a village by the shore, enter from the main entrance and you will see on the floor a blue wool block and some orange wool blocks, dig down one of the ORANGE wool block and you'll fall down in a pit with a dynamite trigger pad on the center, without stepping on it, search the 4 chests that are mounted on the walls and you will find a diamond there, just take it in your inventory, toss it on the floor and pick it up again for the achievement.
Now, if you want to get
dig some sand blocks and return to the surface, get back to the village that you spawn in and exit and save the game. Now connect a second controller or call a friend and throw the diamond to your second player to pick it up.