I think you did everything right for this picture. Great depth of field, great sharpness and proper exposure. The one thing going against it is the sepia. If this were entered in any other challenge I would have scored it higher. But the sepia really turns the diamonds yellow and that's not what one wants to see in a diamond ring.
I like the overall tones in this image, although I'm not sure it actively *improves* the subject. I feel this image is very sharp, but the empty space on the left and right makes me think a tighter crop might improve this composition.