The Sapphire Amulet of magic is an item that provides the highest magic bonus of any amulets in the game. You can make this amulet by taking a cut sapphire, a gold bar,and an amulet mould to a furnace and crafting an unstrung sapphire amulet. Then, you can string the amulet with a ball of wool. The strung amulet can be enchanted at level 7 magic.
You can also purchase it from Davon's Amulet Store.