Just around the engagement period/early marriage. Kate wore these formal-event earrings a few times.
In May 2012.
At the wedding of Sarah Louise Stourton and Harry Aubrey-Fletcher in January 2011.
At the wedding of Rosie Bradford and Harry Meade in October 2010.
Here is the closeup:
It appears to be a halo-type drop earring. There is a large gemstone surrounded by smaller stones. The post is a stud that attaches to the earlobe. There is a small stone fixed between the post and the drop, giving it a slightly longer appearance.
There is speculation among the Kate’s fashion community about whether these are real diamonds or cubic zirconias. I personally think they are cubic zirconias. I believe Kate would wear these more often if they were made of real diamonds. It doesn’t make sense for her to put these away if they are fine jewelry. She mostly wears fine jewelry now. Also, from what I have seen in photos so far, these don’t sparkle the same way diamonds sparkle.