Great question!
We normally show pearls on white background. It's a combination of showing the product best overall vs. total online experience.
Having said that, some of our partners show them on gray/shaded backgrounds, and it has not hurt their performance.
instagram pro
I think in the end it's the quality of the photography that matters most... e.g. a lousy picture on white is worse than a great picture on black... a 20 megapixel image vs. a 12 megapizel image is better... regardless of the background. Right now reshooting a lot of product at higher resolutions/sizes is a priority for us. I'm confident that doing so - regardless of background - will result in better "reality" based imagery/shopping experience for the customer.
FWIW, my own personal preference is black, or dark red... e.g. darker is better. I think it provides best contrast and shows off pearls to best advantage. But given that I see pearls shown overwhelmingly on white background, I have to say my personal preference does not seem to predominate...