Every surface on a material is unique on how it treats the light and uses it to create shadows, mid - tones and the highlights. This group of gems are focused on the materials where the light passes through them, meaning that the light can pierce through the surface and can create subsurface scattering. Not having an object to look at, it is very helpful to do a sketch with few shading strokes just to get the rough picture.
Sketch:
Cheers,
VoodooLizard