Matt Dunkle has cut a number of dark Namanga garnets. His suggestions are:
Use anything from 37.5° to 39° with a shallow crown (I seem to remember using 35° a lot). Also, if you can find it, use a 7 main SRB cut – anything that affords larger facets. Hex designs should work well, as long as they are not too complicated (less than 60 facets). You want broad flashes that return large volumes of light.