" Not sure if this is a Mental Ray or not..."
-check your rendersettings LOL

You probably have lit the scene with FG or GI, so it's probably the quality of that or the number of sampels have to be higher.
Regarding the dark areas in the diamond this has to do with the amount of photon's you have in the scene up the number of photons emitted by your image/light and maybe even up the photonintensity by a hair