For those unfamiliar with chealting agents here is a definition:  Chelants are "chemicals that form soluble, complex molecules with certain metal ions, inactivating the ions so that they cannot normally react with other elements or ions to produce precipitates or scale. The problem with chealting agents on stone is that the iron is part of the stone and they are generally ineffective.  However, EDTA is a chealting agent that can be purchased from most chemical supply companies or for small quantities you can find them at scientific supply companies. Simply do a search on the web to find them.

Dr. Fred