This phenomenon is called subsurface scattering.
Subsurface scattering marble.
Subsurface scattering is critical for creating materials for all different kinds of surfaces like paper marble wax and most importantly skin.
Subsurface scattering works by simulating how light penetrates a translucent surface like a grape for instance and is absorbed and scattered and exits the surface at a different location.
Skin milk marble and wax are the materials which most commonly owe their look to sss although this is an optical phenomenon that is present in virtually all non metallic materials.
This means that light scatters inside the material before being either absorbed or leaving the material at a different location.
Subsurface scattering is important for realistic 3d computer graphics being necessary for the rendering of materials such as marble skin leaves wax and milk.
All non metallic materials are translucent to some degree.
Marble is a typical example and this is the reason why most subsurface scattering demos feature marble statues.
Light scattering 3d computer graphics marble real time computer graphics shadow mapping.