I used an mib_mix_color to layer mental ray mia materials. The one on the bottom has a glow from mia_light_surface and the top is just black. the sphere has a displacement map, and the mib_mix_color uses a cavity map to drive the weight of the shader on top. Hot lava glows underneath while the crusty part on top stays dark.  The extra glow on this one was added in Photoshop.