OK, the official word (and a warning) on opaque stones, from our head judge:
There is nothing in the rules that says that the stone being cut has to be a top quality gem. The judging process does not concern itself with the interior of the stone. It only judges the quality of the cutting, the meet points, the facet edges, the polish and the dimensions. Therefore it should not be a problem if a contestant wants to submit an opaque stone. Word of warning though to the cutter: If a flaw in the stone breaks the surface of a facet it will result in a penalty. It is far easier to see flaws in a transparent or translucent stone than it is in an opaque stone.