I believe the G2 has a macro mode (look for a flower icon). It might take you a few tries, but you should be able to shoot through the IS with the camera in that mode, from a short distance. I found the easiest thing was to find a holder for the diamond (because otherwise I would have needed a few extra hands). I drilled a small hole in a clear plastic cap -stole that idea from someone else here on PS- and put it on my light table. A white piece of paper would work too, if you shine a light directly on it.
Then you can use whatever image editing software you have to crop it out. I'll post a pic of my setup, it's kinda hard to explain...