Diamond has a different complicative structure where we can see C-C BONDS ARE VERY STRONG AND HAS A TETRAHEDRAL ENVIRONMENT
and also ,generally we know that diamond shines, here there is a unique phenomenon taking place here, that is total internal reflection
TOTAL INTERNAL REFLECTION:
When the angle of incidence is greater than critical angle, the light ray gets reflected into the denser medium back without undergoing refraction,this phenomenon is called total internalreflection.
so coming to the critical angle "IT IS THE ANGLE OF INCIDENCE AT WHICH THE LIGHT RAY TRAVELLING FROM THE DENSER TO THE RARER MEDIUM,GRAZES THE INTERFACE IS CALLED CRITICAL ANGLE FOR DENSER MEDIUM".
therefore, the critical angle of diamond which is 24degree is nothing but the angle of incedence for which instead of refraction you can see reflection