Mercury

Mercury, often called quicksilver, was a heavy, white-silver metal.

History
The age-old Netherese Mines of Dekanter were rich in mercury, as well as other metals, and the steady supply met the Netherese arcanists' demands. After the fall of Netheril, the mines became overrun by goblins but still had a rich abundance of metal.

A companionless duergar once believed he had come across a whole pool of mercury, but it actually turned out to be an Abyssal gate. After touching the otherworldly gate, he fell under the control of Orcus, who used him to conquer Deepearth.

Applications
The divine evocation spell flame shield required a drop of mercury as one of its material components. The extremely powerful arcane spell chain contingency needed 500 gp worth of quicksilver in order to be cast. Other arcane spells, such as Tenser's floating disk and maddening darkness only required a drop of the substance.

According to the imp Cespenar, liquid mercury could be used to enhance the legendary Angurvadal sword known as the Stream of Anguish.

Trivia
Some dragons, known as mercury dragons, had the ability to turn into liquid quicksilver, a form that allowed them to easily pass through difficult terrain.