Thaumium Tools and Armor
Thaumium is a wondrous metal gained by infusing iron with most kinds of vis crystals. If you have some of the Forge mods installed like RedPower, IndustrialCraft or Forestry you can also infuse bronze, brass or silver to gain Thaumium.
Thaumium can be used to create armor or tools like normal iron and the tools or armor created lie about halfway between iron and diamond in terms of efficiency, durability and strength.
Where Thaumium really shines however is when an Enchanting Table is used to enchant them. Thaumium is capable of absorbing vast amounts of magic, making it extremely easy to enchant.
This strange, dark metal seems to suck in the light around it. You can also create simple armor from these ingots that has similar properties to diamond armor, but is more durable and easier to enchant. You can't craft tools... yet.