Azymus Ore

Azymus Ore is an ore currently found in The Nether, however it will one day be in all dimensions. It drops Azymus Powder, which is used as fuel for the Purifier.

The Nether:

It is found in places where Netherrack spawns (That means just about anywhere).

It is not necessarily "rare", however its close resemblance to Netherrack and the fact that it is imbued in it, makes it hard to spot.