These earrings match the necklace Kate wore during the engagement announcement.
These earrings look to be from the same collection as the Elsa Peretti necklace. It’s been difficult to receive absolute confirmation that they are Elsa Peretti designs (because I think these are from a previous collection and no longer available for sale), but I would bet money they are from the same designer.
A bezel diamond set in yellow gold holds the post earring to the earlobe. Beneath this dangles a sapphire cabochon set in yellow gold.
Here they are again on Kate during her arrival in Canada (July 2011).
And at the Summerfield Community Centre in Birmingham, UK (August 2011).
I’ve noticed that she hasn’t been wearing these as much nowadays. She’s been relying on her sapphire and diamond drop earrings when she needs blue toned earrings to complement blue outfits. I would love to see these on her again!