Technically crema forms on espresso, not coffee. Crema is the layer of light colored foam on top of an espresso shot. Crema only forms when a shot of espresso is pulled correctly. The composition of crema is the natural fat content coffee grinds, which is emulsified when a shot is pulled. During and after the shot is pulled and the espresso is in the cup, the crema rises to the top, forming a thin layer on the espresso.