Treated gemstone is one in which a physical property, usually color, has been modified to give it more value. A change in color does not necessarily mean an exchange for another, as it may just be an improvement in the natural color. In cases where the gemstone has only an improved property, it can be said to be an enhanced gemstone, a particular case of treated gemstone.
The Irradiation process is exposing a gemstone to the effects of radiation to change its color. There are several radiation sources used for this purpose. The use of X-rays requires equipment that is easy to obtain, but provides low color uniformity, little penetration into the yolk and, therefore, it is not a commercially viable process. Colorless or pale yellow sapphires under X-ray action turn yellow, similar to topaz.
http://www.cprm.gov.br/publique/CPRM-Divulga/Canal-Escola/Gemas-Tratadas-1277.html