I like the concept. I imagine the colors didn't show up quite right on the shell. Its not in focus and the darks are noisy. I would coil up the necklace so its not running out of the frame. Use an overhead, diffused light. Also use a black velvet to absorb more light giving more solid black background. From the reflection on the pearls It looks like your using 2 light sources of different temperatures. It could be that one is diffused and one isn't giving a different color. Keep on trying!