Better known as Fool's Gold, pyrite is an iron sulfide compound with a gold-like appearance, hence its nickname. Unlike gold however, pyrite often forms in cubic crystals, an expression of the mineral's cube-shaped atomic structure. 

This specimen is a Peruvian Pyrite Cluster, measuring 2.84". It might not be worth its weight in gold, but this mineral has been valued since prehistory when pyrite was used as a fire starter by Neanderthals and our other human ancestors.