After a lot of tinkering with the node editor, I think I have finally come up with a realistic-looking procedural texture for pearls. These pearls have subsurface scattering, layers of sheen, and emit subtle colors in the light just like real pearls do.
These look more like freshwater pearls--the sheen is much softer and less reflective than fancier akoya pearls. You can easily change this. For akoya pearls, turn down the roughness of the glossy shaders and make the colors throughout the pearl a bit bluer.
Note: For these pearls to appear realistic it is best to have good lighting. The nature of the material (lots of glossy and subsurface shaders) means they don't look like much in bad light and shine best with more than one light source. Also, this is public domain, but I would love to hear of anything you make using this material!