Answer
No, it isn’t diamond.
Work Step by Step
Apply equation 24–6a to find the polarizing angle.
$$tan\theta_p=\frac{n_2}{n_1} $$
For an oil–diamond interface (1 = oil, 2 = diamond)
$$\theta_p=tan^{-1}(\frac{2.42}{1.43})=59.4^{\circ}$$
This is different from the observed angle.
