The Cannon ball is an item created and used as part of Between a Rock.... Players make it by using a gold bar on a furnace, with an ammo mould in their inventory, granting 37.5 Smithing experience. This makes only one large cannon ball, instead of the four cannonballs that are usually made from a steel bar.
This item is used to shoot through the rock; every other item bashes against the rock, but this one goes right through it as if it were not there. That is why players need to make the gold helmet later to shoot themselves into the rock to find the golden cannon ball. The first thing they will see is gold rocks, a lot for later mining.
Note that the players do not actually find the cannon ball; it will become lost within the rocks.