Onyx Screw of Boomboom is obtained by killing the Rigal Bomb found in the Crystal Quarry.
The screw normally drops when he is killed. The screw is considered to be a Melee class item. Since the Onyx Screw of Boomboom is a blue named item, it cannot be traded in-game. It can only be traded through the exchange market. It is highly recommended any party's killing the Rigal Bomb to set the party's distribution to "Free" and have any Melee class obtain the item.