Thaumium is a magical metal created using a crucible with eight Metallum and four Praecantatio. Ingots are used to make a variety of thaumium weapons, as well as special types of magic blocks, such as Arcane Stone Blocks.
Think of it as a base magic metal used to make other magic devices.
Before it can be crafted, the player must first research Basic Transmutation, and then Thaumium. (See research details)
Here's a video about Thaumium:
Thaumium is used in the crafting of the following items: