We saw this necklace once at the children’s play in Kensington Gardens (June 2012).
It’s a chain bib-type necklace made of silver, black beads and what appears to be crystal beads. It drapes down in a small bib whose longest point reaches the sternum. The collar of the necklace consists of sparkling black beads along the length of the chain.
Here it is on Kate.
I believe this is costume jewelry. I haven’t been able to find other photos of this necklace, so I wonder if this was purchased after her wedding.