My experience is that when you photograph pearls, you should first carefully choose the background, which may either absorb or reflect too much of the pearl beauty. With exotics light background was definitely better, with white pearls - a dark one.
Another thing is the flash: it depends, but very often making a photo in a daylight is better. Although not always, you have to experiment. If it is with a flash, somethimes it depends upon the combination of the flash and the lamp, and which side reflection goes. It also helps to look if shooting 'down', straight against the background, is the best way - sometimes it is better to take a side shot.
One more tip: on good cameras there is a function to make a 'pointed' focus on the subject in the middle, but you have to wait a couple of seconds until it gets tuned, so that you see it in the viewer and only then press.
Hope it helps a bit
Olga