[Raymond Motion Pictures]"How can I get the diamonds to sparkle as they turn?"
Very similar to something I did, turntable and all. I tried star and chromatic filters over the lens, which looked okay, but not great. The emotional resonance of seeing something sparkle somehow exceeded the actual sparkle that the camera recorded.
In the end I cheated and did this in post. If you use After Effects there are a number of filters available (for extra $ in some cases) which will allow you to dial in a sparkle to your heart's content.
I've done tons of jewelry spots; it's not too hard but takes patience to get right.
I've typically used a combination of large reflective surfaces for the precious metal settings and small hard sources for the sparkles. Some people use a single source and several mirrors to reflect into the facets.
You can try star filters, but they can get cheesy very quickly.
One note, most turntables are not smooth enough when you are shooting such extreme close ups; a lead screw is often necessary to reduce the screen bounce. Many rental facilities have them.
My nifty turntable worked great! I hung the rings upside down, dangling on a stiff wire (mandolin string) - looks great on flipped video with highlight sparkles inside a light-tent. Client was pleased with rings filling only a quarter of the screen and his logo and ring description taking up the rest of the space...the ring via my rig seemed to be floating in space. Yeah, I lucked out..
Lesson learned...The challenge to filming jewelry is about lighting as much as it is about optics - read: macro lenses.