Pyrite

Pyrite, first discovered in a liquid state inside Ulduar is a rare mineral found when mining in Cataclysm.

Source
Pyrite is mined from Pyrite Deposits and Rich Pyrite Deposits (Both require 525 Mining).

Locations
Pyrite is found only in the highest tiers of Cataclysm zones, those being the Twilight Highlands and Uldum (more frequent in the former). Rich Pyrite Deposits are only found in Tol Barad (both in the peninsula and the battleground-area). Both types are rare spawned nodes off Elementium, though they appear to occur more frequently than previous rare nodes.

Trivia

 * In real life, Pyrite is often referred to as "Fool's Gold" and is an iron sulfide. It has little practical use in modern times, though it still remains popular in jewelry due to its shine.
 * Interestingly enough, the ore in game is rainbow colored while the actual bar and nodes are both gold. In liquid form, pyrite appears to turn blue.
 * Pyrite has also been called "Baked Potato" or "Baked Potarite" or other variations, due to the node's resemblance to a baked potato.