Reflections & Refractions:
The picture above are two diamonds made for an external client. These diamonds aren’t using a texture image. The shading only uses light reflections and refractions. The whole diamond “feel” is purely created by the lights and reflections and light bounces of the the mesh. This is in style of how real diamonds get their aesthetics and look & feel. The diamond consist of two meshes of the diamond shape within each other. This is so the diamond calculates the light it bounces of the edges and polygons (the outer mesh) and the light that bounces and reflects inwards (the inner mesh).
If you combine these two meshes they create a network of light reflections and refractions, simulating the effect of a diamond.