Adamantite is a very rare trading item used to make Adamantite Armor and weapons that grant a large bonus when three are equipped. There are only three ways of obtaining it - through Story mode (only two pieces possible), a very rare drop in Duel Colosseum, and by finding a Pink Tail. Pink Tails can only be found in the "Lunar Whale" Duel Colosseum and are rare. It costs 56 medals to buy. To increase your chances of finding it, raise card luck as high as possible, have the "Thief" ability card, and just keep battling opponents. Once you find one, Adamantite should be unlocked in the store, costing 2000G and one Pink Tail.