Make a circle, any size. Make your foreground color black, and the background color whatever color you want the gem to be, bright colors work best, I've chosen a bright blue here.
Flood fill the selection.
Make a new layer, right click on the layer control panel and choose New. Set the foreground color to white. Click the spray can tool. Spray on the dark part of the gem. This is easier to see in the following example than it is to describe.