Kate wore this necklace to a polo match in June 2006.
Here is the closeup:
It appears to be a chunky-style lariat Y-shaped necklace. Those could be rose quartz crystals or just pink-colored beads strung on the necklace. There are silver beads laced between the pink beads.
It appears to be a very long necklace, at least 36 inches long. I estimate it could be as long as 40 inches.