  We investigate the effects of Raman and Kerr gain in crystalline
microresonators and determine the conditions required to generate modelocked
frequency combs. We show theoretically that 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.
