Pyrite

Pyrite, also known as iron pyrite or fool's gold, was a golden-hued mineral  containing iron.

Description
With its yellow-metallic sheen, pyrite resembled gold to the uninitiated eye, but was much less valuable than the precious metal. Deposits of fool's gold however did sometimes contain a certain amount of real gold. Pyrite also appeared in small quantities within lapis lazuli, giving the blue ornamental stone its golden flecks.

Trade
Iron pyrite was commonly available at many towns and markets. It was sold by Aurora's Emporium in the 1360s DR.

Uses
Pyrite could be smelted to yield the iron it contained.

Pyrite was one of the reflective materials used by the dao to decorate their capital on the Elemental Plane of Earth, the Sevenfold Mazework, and help with its illumination.

Magical Uses
Iron pyrite was common stock in a reasonably equipped wizard's laboratory.

It was the material component for the spells Leomund's trap,  and  cone of earth.

The conical hat, an Imaskari relic dangerous to the wearer's mind found in Solon around 1360 DR, was made of iron pyrite.

Religious Significance
The discovery of iron pyrite was sent as a portent by Waukeen to her faithful, as well as a sign of displeasure by either Dumathoin or Vergadain to their dwarven followers.

Trivia

 * The Iulutiuns used so-called spark stones resembling pyrite to light their fires in the cold of the Great Glacier. These could be found mostly in the Novularond.
 * According to some tales, the Gilded Nugget, the founding price of the gnomish nation of Songfarla, was an enormous piece of iron pyrite.