Walking here with King Frederik and Queen Ingrid of Denmark is one of the queens at the Greek wedding who is still a queen today: Queen Sirikit of Thailand, wife of King Bhumibol Adulyadej.
Sirikit has one of the biggest jewel boxes around, and she has many tiaras that look very similar to one another, so much so that I'm ultimately guessing a little about this tiara's identity. Based on the black-and-white photo, I think this is Sirikit's Diamond Tiara. Love the way this one cascades down the sides of her head. Beautiful!