Ferric OXIDE, also known as rust, is a chemical compound composed of iron and oxygen. It is an important component of the Earth's crust and is formed when iron is exposed to oxygen and moisture in the environment. Rust is a reddish-brown color and is often seen on metal objects that have been left outside for long periods of time. Rust can cause corrosion and weaken the structure of metal objects, making them more susceptible to damage.