If you take a look at an opal photograph on the web or in a book, it rarely does justice to the actual gemstone.
That’s because the play of color is really only visibile when the gemstone is moving.
Usually, the color is captured but the play of color is missed.
One method to capture an opal at its best is to submerge it in water. (Of course, you’d only do this with real opal, not doublets or triplets). This results in all of the opals showing simultaneously.
Take photos of dark stoneson a white background, and white or crystal opals on a black background. Increase the exposure a little bit to compensate for a lack of light.
Of course, the best way to photograph an opal is with a video camera, so that the complete play of color can be captured.