When light neither "passes through", nor "bounces off".
Many real-world materials are neither transparent nor opaque, including milk, marble, and other stones. These materials admit some light beneather their surface, where it is bounced around for a bit, only to re-emerge elsewhere on the surface. The effect of this "subsurface scattering" is the difference between the appearance of chalk, or liquid paint, and milk.
The search for realism continues. For the very latest, look up the proceedings of the mid-summer SIGGRAPH (Special Interest Group on Graphics) conference of the ACM, or go to one (along with thousands of others), or check out this website for more examples and information.
Last updated: April, 2014