Argon is the third most abundant element in the Earth's atmosphere, making up about 0.93% of its volume. It is a colorless, odorless, and non-flammable gas. Argon is produced naturally by the radioactive decay of potassium-40, which is found in small amounts in all rocks. Argon is also produced by the industrial separation of air, which is done by liquefying the air and then distilling it.