I found one photo of Kate wearing a long white bead necklace which appears to be knotted at the front.
The editors at Waity Katie’s blog believe these are pearls she’s wearing here. I think these are white beads due to the shape of the beads and the lack of iridescence.
Here is the close up:
It looks to be a 25-30 inch necklace which was knotted at the center to gather it. After being knotted, it takes on the appearance of a lariat. The beads are placed on the string a few millimeters apart, and I would guess there is no clasp in the back. This is most likely a slip-on necklace.
This photo was taken in 2007.