While she was dating William in 2006, Kate wore these statement earrings.
I believe they are costume jewelry.
Here is the closeup:
It’s a pendulum-type pendant, with a sideways “D” at the bottom. Two string attachments flank the edges of the pendant and allow it to swing from what I think is a French hook. It does not glisten the way silver does, so I think this is a white pendant and not a silver one.
I believe I have seen her wear this at another time. I will update with more photos when I find some. You may also send me photos if you find any.