Elementium Ingot

Elementium Ingot is the hardest metal known to Azeroth, even moreso than diamond. Very rare and difficult to find and process, it is used in the creation of extremely powerful weapons and armor. The ore can only be found in Blackwing Lair and the materials and knowledge required to make use of it can only be obtained with the backing of a raiding guild.

The ingot can be used for several epic-level quests or to create Enchanted Elementium Bars, which are used in the creation of Thunderfury, Blessed Blade of the Windseeker.

Source
Elementium Ingot drops from Blackwing Technicians (goblins) in Blackwing Lair.

Mining

 * Smelt Enchanted Elementium

As a quest objective

 * Quest:The Good News and The Bad News
 * Quest:Imperial Qiraji Regalia
 * Quest:Imperial Qiraji Armaments

Background
This metal is extremely rare, found only in some of the most remote regions of Azeroth. It is believed by archaeologists to have been formed during the prehistoric reign of the Old Gods, in places where there were high concentrations of elemental creatures. In fact, elementium isn't native to Azeroth, and is found only near high concentrations of elemental creatures because they bring it from the Elemental Plane where it is actually quite common.

The unique properties of elementium allow it to absorb and channel elemental forces. Skilled smiths temper elementium in blistering forges, adding to the quality of the item. All such objects are masterwork quality and are immune to cold, fire and electricity damage. Elementium is the hardest known metal. Elementium is so heavy that to craft armor from it would leave the wearer virtually immobile, so the metal is usually only used to craft weapons. Elementium has no market price. It's too rare to be available. It's not something you buy-it's something you go get.