First off, SPOILER ALERT!!! With that out of the way, I have found a good way to acquire lots of diamonds in Dead Island. Read on to find out how…
First, you have to go to City Hall and speak with the Mayor. After a cut scene and a bit of story line, you’ll go off to the Mall and do a quest there. When you have completed that, you’ll head back to City Hall, only to find that it has been attacked and is overrun with zombies. This is where you can start farming diamonds.
As you come into City Hall from the direction of the Mall, you’ll start in a room with a work bench in front of you and a door on the right. Go through the door and up the steps, being sure to avoid or kill the zombie there. Continue up another two flights of stairs, until you see two lockers in front of you and a door on either side. Go through the door on the left and kill another two zombies. There will be a door on your right as you enter. Further in the room, on the left, there’s yet another door. Break this door down and look under the desk. You will notice a safe that you can open, which contains a single diamond.
Now, this doesn’t sound like a lot, but if you go back the way you came, all the way to the Sewers, and then re-enter City Hall, the diamond will respawn. Simply lather, rinse, and repeat, and you’ll have a bunch of diamonds in no time.
You can choose to either just sell them for $1500 a piece, or use them to mod out your weapons, it’s up to you. Hope you find this useful. Enjoy.