Nice idea and the edit works well. This is interesting in that the surrounding rainbow actually draws your eye to the pearl. On my monitor I can see a reflection of a rainbow in the top half of the shell.
Aside from the pearl looking a bit funky to me with the yellow coloring, I like the background. Little bit of noise/grain in the shadow, but overall, good entry.
You are clearly going at a different approach for rainbows but it seems a bit of a stretch for me in this challenge. From a photograph perspective, the image itself is pretty well done - the details in the oyster are nicely captured.