Mark is right that just pointing lights at the subject will be disappointing. There are any number of aids on the market, some are translucent cones with a hole for the camera, others are more like half globes. By directing your lights onto the outside you create a large, specular, soft source on the subject on the inside. You may be able to create the same effect yourself by tenting the turntable with a “silk” or other translucent cloth.
Another highly effective way to light glass – which optically is what jewels are – is having them on a white plexiglass suface. The key (or strongest) light is from below and top or side light is used for balance.
There really is no substitute for a LOT of experimentation. Want to really impress your client and show them just how hard this stuff is to light? Spend a lot of time experimenting on your own, then step-by-step re-create the parts of the experiment that worked with your client watching the monitor.