If a beam of light strikes a diamond at an angle of 45 degrees
What is the angle of refraction?

Use Snell's law: \frac{\theta_2}{\theta_1}=\frac{n_1}{n_2}
where \theta_1 is angle of incidence, \theta_2 angle of refraction,
\frac{n_1}{n_2} relative refraction index between air/vacuum (from which incident beam comes) and diamond.