Why diamond is so costly?

Diamond is just an allotrope of carbon still so expensive because of the art work performed on it,

Whenever any light enters a piece of material it tries to travel through it or cross it but some of them suffer total internal reflection (TIR) when the incident light crosses the mark of critical angle,

According to this the light is not able to come out of the material and it suffers multiple reflection within it's boundaries.

For a well crafted diamond the value of critical angle is 22.4