Iron is, by mass, the most common element on Earth, at 32.1 percent, just ahead of oxygen, at 30.1 percent, and it is the fourth most common element in the earth's crust, after oxygen, silicon and aluminum, at five percent. Pure iron, which has a silvery-gray and, when smooth, a mirror-like surface, is rare in the earth's crust because it reacts readily with oxygen and water to form oxides.
Iron alloys, including steel, stainless steel, cast iron and special steels, are by far the most commonly used industrial metals, because of their great strength and low cost. Iron is also an essential element for some proteins and enzymes that are required by living organisms, and, for example, an average adult human body contains about four grams of iron.