The gold helmet is a helmet you must smith during the Between a Rock... quest using 3 gold bars. You get 30 Smithing experience for making it. More than one helmet can be made by using the drop trick and casting Telekinetic Grab on the helmets you've dropped on the ground. You must wear it to use Dondakan's cannon to enter the massive Arzinian Mines, which are found in the mines south of Keldagrim.
During the quest, once in the mines, the helmet will begin to get heavy. Because of this, a player may only stay in the mines for eight minutes before the helmet becomes so heavy that they are magically forced out. After the quest, the helmet will not force players out, although it must still be worn to enter the mines. It is the heaviest piece of equipment in the game.
To leave the mines, a player must remove the helmet. When this is done, they will automatically be teleported outside. This costs the player one inventory space, so some players log out instead; if a player logs out in the mines, they will automatically be teleported outside.
To make the helmet after the quest, have 3 gold bars and a hammer in your inventory and smith it on any anvil (option won't be available if you have one in your bank already).