The Black Eye skull is a gold skull. It is the first skull on the second level, Crow's Nest, and is in the same room you start in after the cinematic, high above your head atop a large pipe.
Getting to it is relatively easy. Walk to the end of the room, up a short staircase, and turn around and look for a green shelf. Jump up on that, up onto a girder, and from there up onto the large pipe. Walk back the length of the room to the end of the pipe nearest the video screens and claim this skull.
Iron is Halo 3's second skull and the second skull on the first level, Sierra 117. It is the game's first gold skull.
As a gold skull, it gives you and your cooperative play teammates the Iron achievement if any player picks it up during the game. After picking it up, it can be activated in the campaign settings page of the pre-game lobby.
When activated, this skull will force all cooperative players to reload the previous checkpoint if any of the players die, and will force a solo player to repeat the entire level if he or she dies.
Like its Halo 2 counterpart, when activated the Blind skull removes all interface elements from the screen. It is a silver skull, and provides no achievements or campaign scoring multipliers.
It is the first skull encountered in the game. It is out in the open, at the end of a rock outcropping overlooking the river to the far right of the pool where the Phantom drops off the first Grunt and Brute reinforcements during the first encounter of the first level, Sierra 117.
The Grunt's B-day skull is pretty easy to get. First you beat the game on any diffuculty level, but easy.Then go to the second level on normal or higher. Go through the level normally till you get to where the drones are passing back and forth throught the pipes(right before you jump down to meet the Arbitor).Stop right where you're supposed to jump down to the Arbitor and look down. Right below you , you will see a green arrow pointing backwards. Jump on the arrow and turn around. You will see a hidden room with the skull in the middle.