Iron in which a large percentage of the carbon content is in the form of graphite flakes. Traditionally referred to as “Cast Iron”. The graphite flakes cause it to have low shock resistance, but high damping ability. It has a gray fracture. Gray Iron is by far the oldest and most common form of cast iron. As a result, it is assumed by many to be the only form of cast iron and the terms "cast iron" and "gray iron" are used interchangeably. Cast iron containing graphite in flake form and typically consisting of 2 to 4 percent carbon and 1 to 3 percent silicon. Gray iron is widely used for engine components in automobiles and trucks. See Cast Iron.

Gray iron or grey iron was the original "cast iron". It is relatively easy and inexpensive to make. Compared to the more modern engineered irons, gray iron has a lower tensile strength and lower ductility.