One of the biggest reasons for the yellowing of marble is the oxidation of the iron naturally present in its structure. When marble is exposed to plenty of water or cleaning products containing various chemicals over time, the iron in its structure is oxidized, which may cause yellowing on the surface. These yellowings are also known as rust stains on marble. In more serious cases, brown spots may also appear. Other factors that may be the source of yellowing in marble can be counted as wearing out over time and improper cleaning.