A big thing is the color of the back ground the item will be resting on or highlighted against. if it is a more transparent colored stone, go fo the color that will make the stones color stand out. Blues for colorless diamonds, blacks for rubies, aquamarines and morst tourmalines. Yellow for alexandrites, shappires go with a nutral light color background. If you have an opaque stone such as like a pearl, darker colors tend to show off the luster. If it is a smaller stone, it can get lost in a busy background, if it is a large stone, be really careful of the fill lights they can make them look funny.
If you have a ring, use a bit of that clear stuff they use to adhere posters to walls to hold it in the position you want. Props to lay the item against are good, just be mindful of the texture patters of the background. You don't want to have you jewelry disappear in the texture.