Pyrite

Composed of iron disulfide (FeS2), this mineral has hues of gold, but not its value, hence its nickname: "fool's gold".  

The skeletons of the Iguanodons of Bernissart were encrusted with it. Covered by marsh clay, the carcasses of these dinosaurs decomposed under the action of cyanobacteria. The acid released from this reaction interacted with the iron contained in the clay, producing pyrite which gradually filled the numerous cavities in the bones.  

Its name comes from the ancient Greek puritês lithos, "fire stone". Prehistoric humans used pyrite to light fires by striking it against flint, producing sparks that ignited dried tinder and in turn ignited twigs and dry grass...