Gold Skull
From Halopedia, the Halo wiki
In Halo 3, there are two types of skulls: silver and gold. When you collect a gold skull, you obtain an Achievement for collecting that skull, worth 10 Gamerscore. You also get a score multiplier for the campaign meta-game if you play with a gold skull on. A list of gold skulls in Halo 3 can be found here.
