Q: I would like to cast something into molds (besides plaster of Paris) that would end up looking like white marble. Should I use polyester resin or what?
A: If I were you I would avoid the use of polyester resin—this material is insidiously toxic, and seems to have shortened the lives of many sculptors. There are newer types of resins out there, such as urethane and epoxy resins, which have similar properties but are not as nasty. Both can be mixed with fillers for an opaque white appearance.
Another alternative is concrete; you can use white cement, which can be mixed with marble chips for a terrazzo effect—this works better if the surface of the casting is ground down to expose the stone. Or you can investigate some of the tech plaster (gypsum cement) based systems which are available, such as hydrostone, hydrocal, and Fortron MG. These work more or less like plaster but set up harder and tougher, especially if reinforced with fiberglass or other similar materials.