Mithril Ingots are a green-colored metal and are obtainable by smelting Mithril Ore in a Furnace. They are used for several things; first of all, they can be used to craft all weapons from Tier 8 (Mithril, obiviously). Second, they are most likely the best repair material, as it is hard to get to the Mystery Dimensions, but Mithril Ore isn't rare at all and it repairs 526 uses. Diamonds still repair more, (insta-repair anything) even tough you'd be better off keeping those for other items.