Kate used to wear this silver ring with an oval center stone before her marriage.
Here is a close up of the ring.
My best guess would be that this is sterling silver with a light colored stone. Possibly moonstone or quartz, but I’m not sure..The silver shines brightly, which means that it’s not silver plated.
Kate was seen wearing this ring often during her early and mid-twenties. The above photo was taken at Englefield House Polo in June 2006. You can see the ring on her right hand (middle finger). On her left hand it appears she is wearing the gold ring that many people believe William gave her.
I believe she is wearing this ring in the photo below (taken in 2007).