How does total internal reflection explain why a diamond sparkles when light shines on it?

It is because of the trapped rays inside the diamond that undergoes total internal reflection

Explanation:

Total Internal Reflection (TIR) occurs when a light ray passes from denser to rarer medium such that the angle of incidence is greater than the critical angle.

Critical angle is that angle of incidence for which angle of reflection is .

Moreover, diamonds have small critical angle and higher refractive index and they are cut such that the light inside it is always incident on the faces at angles more than the critical angle which results in continuous TIR and when light passes light passes from any face, a whole of it comes out making it sparkle.

their crystal patterns have been disrupted by stress

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.