We investigate the effects of Raman and Kerr gain in crystalline microresonators and determine the conditions required to generate mode-locked frequency combs. We show theoretically that a strong, narrowband Raman gain determines a maximum microresonator size allowable to achieve comb formation. We verify this condition experimentally in diamond and silicon microresonators and show that there exists a competition between Raman and Kerr effects that leads to the existence of two different comb states.