Diamond is one of the hardest material. We know that it's an allotrope of carbon. A diamond (crystalline in nature) has a three dimensional arrangement of carbon atoms linked to each other by strong covalent bonds. What you've shown a round brilliant cut diamond.
Actually, the secret that's rattling inside a diamond is refraction, total internal reflection (not to be confused with ordinary reflection) & dispersion. The refractive index of diamond is pretty high (2.417) and is also dispersive (coefficient is 0.044). Due to this fact, diamond is an important application in optics.
Consider an ideal cut diamond. I explain according to the figure below. When the light is incident at an angle 1, it refracts inside and travels through the lattice. At the surface which separates air & diamond media, the incident angle 2 is very well above the critical angle (ca) and simultaneously (3 & 4) the reflection takes place at different surfaces of the diamond. Finally, the light refracts out.
The first one shows the mechanism of internal dispersive reflection. The second figure shows the reflections inside ideal cut, deep and shallow cut diamonds.
Note: For total internal reflection to take place, light must travel from an optically denser medium to a relatively rarer medium. Also, the incident angle should be far high above thee critical angle.
There are youtube goodies regarding the topic...