We demonstrate the cooling of a microwave mode at 2872 MHz through its interaction with optically spin-polarized NV− centers in diamond at zero applied magnetic field, removing thermal photons from the mode. By photo-exciting (pumping) a brilliant-cut red diamond jewel with a continuous-wave 532-nm laser, outputting 2 W, the microwave mode is cooled down to a noise temperature of 188 K. This noise temperature can be preserved continuously for as long as the diamond is optically excited and kept cool. The latter requirement restricted operation out to 10 ms in our preliminary setup. The mode-cooling performance of NV− diamond is directly compared against that of pentacene-doped para-terphenyl, where we find that the former affords the advantages of cooling immediately upon light excitation (whereas pentacene-doped para-terphenyl undesirably mases before it begins cooling) and being able to cool continuously at substantially lower optical pump power.