Ivory = SSS + Fresnel Reflection, with a roughness determined by polish and/or coating
Most of getting that to be different than, say porcelain, teeth, or skin is controlling the IOR (water is 1.33, I suggest going below that unless you want really polished or finished ivory), controlling the blur on the reflection (again, depends on how shiny you want it), controlling how strong your SSS is (how far light scatters within the substance), and texture (which would really depend on what you're making).
The _big_ thing is to get a good range of reference images with color neutral lighting of the specific type of material you want to make.
Oops, sorry, just saw the date. Well, I'll post this anyway, in case you need it.