This item is added by Thaumcraft 2 , hence the name "Thaumium".
Thaumium is a wondrous metal gained by infusing iron with any kind of vis crystals. You can also sometimes find it in some Battle Towers /dungeon chests. Making it from scratch is not very efficient as it consumes a iron ingot and vis crystal. It can also be used to make Thaumium tools and armour,Tools and armor made from Thaumium are stronger than iron, but weaker than diamond
However, where Thaumium really shines is when an Enchanting Table is used to enchant their tools/armor. Thaumium is capable of absorbing vast amounts of magic, making it extremely easy to enchant.
Thaumic Infusion Edit
As said before, thamium ingot can be used to craft mid-game armor and tools using sticks and ingot like any other vanilla tools and armor, however they are more durable than iron and support more enchantement than any of the other actually in-game material