It is represented by a blue circle with a skull in the middle. The skull has a clover on its forehead. Behind it are smaller dark-blue skulls.
The Tough Luck skull can be found on Tsavo Highway on Normal or higher difficulty. After you have smashed down the first vehicle barrier, you should end up on a highway, with a CCS-class battlecruiser flying over your head. Look to your left and you will see a pipeline running along the highway. Jump on top and begin traversing it. Jump down to the ground below and walk away from the highway. After some tactical jumps you should see a ledge to your left again. You can get on the ledge by crouch jumping or grenade jumping to where the skull will be for your pick-up.