Gray Iron: The actual composition of the ductile iron is unknown. However, cast irons have greater than 2% carbon, 1 to 3% silicon and the remainder as iron. The dark areas represents Fe3C (cementite) and the light represents Ferrite. Note the ball structures- this seperates gray iron from ductile iron. Also notice the dark balls with the light spots in the middle. This is where balls of cementite used to be- they were ripped from the sample during grinding or polishing, and this is actually all ferrite, but the lighting is poor in the concave area, producing a dark color.