I looked at that photo when I was trying to see if the Duchess had worn the necklace at other times, but could not decide if it was the same or not.

I also tried to find other images from the same event, but although some of the newspapers showed photos of the Duke dancing with Queen Paola, I couldn't find another of the Duchess. I wish we had access to a sharper image!

If the Duchess was wearing it to the Waterloo Ball then it suggests that she owned the necklace and that it was not a borrowed item. Strange that we haven't seen it more frequently. The necklace is a real mystery.