Pearlite is a two phase material with iron and carbon as its constituents. The diagram shows the lamella structure of the two phase system produced by the equilibrium cooling of a eutectoid carbon steel: 
(0.77 wt % C).  The white areas are ferrite, an interstitial solid solution of carbon in bcc iron, and the dark areas are cementite, Fe3C a binary compound of carbon and iron containing 6.70 wt % C.