Thaumium

Thaumium is a material that has been added since Thaumcraft 3. It has the same hardness level as Diamond, and as such a Thaumium pickaxe is able to mine Obsidian.

Thaumcraft 3
Thaumium is crafted using the Crucible. It requires 8 Metallum and 4 Praecantatio.

Flaxbeard's Steam Power
Flaxbeard's Steam Power only adds the plate variation of the Thaumium material. The Thaumium Plate can be created by throwing Thaumium Ingots into a heated Crucible, and then pouring that molten metal into a Mold with a Plate Mold. Six nuggets are used in the creation of a Plate, which makes it unable to be used in other mod's recipes requiring their own Thaumium Plates.