Kate wore this bracelet to the wedding of Rose Windsor and George Gilman in 2008.
Here is the closeup:
The bracelet appears to be made of cultured freshwater pearls arrange in a linear fashion and extending for three rows. There is a metal bar connector on at least one side of it.
She also wore it to the Order of the Garter in 2008.
These are the only two photos I can find which feature this bracelet. I’m a little surprised she doesn’t wear it nowadays. It’s one of her Pre-Engagement pieces which would meet royal standards for appropriate jewelry.
I found one online which looks very much like Kate’s bracelet.