Pyrite

noun

1. The common mineral iron disulfide (FeS2), of a pale brass-yellow color and brilliant metallic luster, crystallizing in the isometric system.
2. (usually as a plural: pyrites) Any metallic-looking sulphide, such as the above, which is the most common.
3. (usually as a plural: pyrites) Any metal dichalcogenide that is isostructural to the common mineral.

   "Copper diselenide can occur both as a marcasite and a pyrite."