Hitting the jewels directly with hard lights alone will come out disapointing, and is the common first-timer mistake for people just begining to try and shoot this kind of thing. You need some white boards reflecting onto the jewels as well to create the specular highlights and define the shape of the gems and the metals by how theey reflect the boards. You can test this by trying to shoot a metal pipe or a glass tumbler or bottle with and without the white card just out of camera shot.
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.