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.
Infusing normal wood with a vis crystal creates Enchanted Wood which is a component of many different Thaumic tools and devices.
Using Greatwood logs in the infusion process is slightly more efficient than using normal wood and results in 5 enchanted wood instead of 4.
It is also possible to enchant Silverwood logs which results in Enchanted Silverwood - a rare and potent component used in certain devices.
Fusing together redstone and coal or charcoal, results in Alumentum which has several interesting properties.
Firstly it can be used as a fuel like normal coal, but it lasts 10 times longer.
It also seems to have the natural ability to absorb Taint making it useful in the construction of filters.
Fusing together redstone and glowstone results in Nitor, a seemingly eternal and sourceless flame. It forms an important part in the creation of Runic Essences. (See Seals)
Nitor has an additional strange property - it seems to contain more Vis than went into its creation.
Combining Alumentum and Nitor in an Infuser creates an Arcane Singularity.
Arcane Singularities are highly energetic and highly unstable. The slightest jar will cause it to explode. The explosions rips open a tear in the fabric of reality which draws in anything nearby and annihilating it.
Triggering an Arcane Singularity has an..., um, interesting effect on the local Aura.
Fusing together spider silk, wool and a Vis crystal results in enchanted fabric.
This acts as a component for several interesting magical items.
Cinnabar and Quicksilver
Cinnabar is a relatively rare crystal found throughout world. When it is smelted in a furnace it produces Quicksilver that has several interesting uses.
Fusing together a gold ingot, a vaporous crystal and a normal piston creates the Animated Piston. While it cannot be placed in the world like a normal piston would (it is far too skittish for that) it does make a useful component for certain magical devices.
By placing 5 pieces of soul sand in a Dark Infuser (acquired via research) you can create a Soul Fragment. Soul Fragments are used by many other thaumic devices that require a measure of sentience or the flickering remnants of living beings to function.
Cinderpearls are a strange, fiery flower found in deserts. While obviously magical no specific use has been found for them yet. It is possible however to grind the pod into blaze powder.
These shimmering flowers are only found amongst the roots of silverwood trees, and they too glow with an inner magical light. They are primarily used for infusing magical extracts.