Those diamond cuts have very limited facets. Same goes for Emerald cut stones. Same reason in real life those stones have “windows” of little refraction. Helps to make your own 3d diamonds with exaggerated facets (adding in Rhino)., and then resizing as needed. Also play with the “gemstone refractive number or IOR setting. Also try to avoid the stone intersecting any metal (solid). Unfortunately I don’t think Rhino Render has a “ignore intersecting objects” in gem materials like Keyshot (maybe it does and I’m mistaken) . If not , wish Rhino Render did have that option. Helps to booleen difference out the metal with the stone. Hope this helps.
