Polarizing angle, also known as Brewster's angle, is the angle at which light should be incident for reflection not to take place.
It is given by;
Polarizing angle = tan^-1 (n2/n1), where n2 = index of diamond, n1 = index of air (= 1)
Sunstituding
67.4 = tan^-1(n2/n1) => tan 67.4 = n2/1 => n2 = tan 67.4 = 2.4023
If the light moves in the opposite direction;
Sin (critical angle) = n1/n2 = 1/2.4023 = 0.4163
Critical angle = Sin^-1(0.4163) = 24.60°