For my set of jury photos last year, I posed necklaces like this
but I have wondered about it being the "best" way photographically.
I thought it looked nice, but maybe I read something somewhere that
said necklaces should not be photographed having ends hanging. I
would be interested in seeing answers to your question as well.
For pendants on chains or neckwires, I have seen many people photo
them hanging in front of a gradient so, in the picture, it looks like
they are suspended in mid-air. I think it looks the best.
Good Luck and I am looking forward to any input from others