We saw these earrings on Kate at Christmas in 2014.
Here is the closeup:
They are made of nine egg-shaped silver discs with black or dark centers, arranged in the following pattern: one disc, two beneath that, three discs beneath the two, then the pattern reverses back to two discs and just one disc at the very bottom. This motif creates a chandelier style.
What is noteworthy about these earrings is that they were worn several years ago by her sister Pippa. So not only do the Middleton sisters share jewelry, they retain their jewelry for many years.
This prompts the question: will we be seeing more older costume jewelry on Kate in the future? If she deems the piece appropriate, there is a chance some of her older pieces will come out of retirement.
Anna of My Small Obsessions believes these to be costume jewelry. This could be true. I think they are more likely sterling silver (rather than silver-plated) because to be owned for seven years or more and not be tarnished most likely happens with sterling silver and not silver plated jewelry.
Update: Marianna of Books, Birkins and Beauty shared with me a photo of her replikate, which is from Accessorize.
There are differences, but it has the same “spirit” as Kate’s earrings.