Actually, if you get the Ragnarok (or Crystal Sword, in the older version), the Excalibur is good for nothing but throwing. To get the Adamant Ore, you need to find the Pink Tail in the Land of Summoned Monsters, then take it to the small cave near the town of Silverton, I believe. You need the hook and hovercraft to get to it. Once you're there, the old coot that collects tails will give you the Ore. Then you go back to the Underworld, and prettymuch straight South, to the smithy's shop. Give him the ore and the Legend, and he'll make the Excalibur.