When you say its more than 4 prongs, do you mean the standard 6? Or rather is it 4 prongs that are SPLIT into looking like 8 (e.g. 8 smaller prongs). I have seen this recently, I think its an older antique style and I love it. Many of the older diamonds like cushion and mine cuts were placed into these settings in the 1920s and 1930s from what I can tell. Some websites sell similar mountings and some sell the entire stone and setting as well.
Some round stones when placed into these split-prong settings can take on the appearance of making a round diamond appear square, depending on how the prongs are set against the diamond.
Anyway tell me if I am on the right track at all about the split-prong, and if you are interested I can post a bunch of URLs with pictures that I have found lately, as I am considering having our jeweler's designer create a setting with the split-prong effect as well.
