becco - thanks for the info. I couldn't find any answer in the maxwell forum on why my default renders, whether 5 minutes or 50 minutes, are extremely grainy. I saw one render in the maxwell forum that showed dispersion in a head-on view of a diamond ring, but not caustics. I am trying maxwell studio, the stand-alone app. dispersion is apparently impossible in carrara. a good display of dispersion and caustics would be a side view of the diamond at near right angles to the light source (from above at a 15 degree angle), in which the caustics and spectral effects are projected on the plane upon which the diamond sits.