Around 2007, Kate wore these double drop silver and green stone earrings.
Here is the closeup:
At a distance, it’s hard to see the gemstone or bead in the center. The earrings consist of two round pendants. Each pendant is made of two round silver circles, the smaller one settled inside the larger one. Suspended in the center of each double-circle pendant is a single green stone or bead. There appears to be silver connectors on four equal outer sides of the smaller circle (connecting it to the larger circle) as well as a small silver connector just above the green stone.
I believe this is silver plated or possibly tarnished silver. The dim contrast of the photo doesn’t show us the true shine of the earrings.