These are Tolkowsky brilliant round-cut stones. I found a file where someone mathematically described this cut for use in this program. This provide the physical shape.

My effort has been focused on the material properties.

I think the diamonds themselves in the image are extremely good. What I need to work on is the surroundings - the pedestal, backgrounds, lighting. For example, the mirror the diamonds sit on is perfect - no blemishes, no dust, no surface irregularities.

Brian