To Cristina Is this the tiara you mean? BTW, at this same evening in Athens, Princess Irene of Greece (King Constantine II's sister) wore what I think it is a diamond collets necklace as a bandeau (head ornament). Unfortunately I do not have any close-up of this. Maybe one of those diamonds rivières we spoke about in the posts below?
Here is a side photo with Princess Catherine and tiara on the bottom right of picture: Group photo: Here is a close-up posted by Martina, as she says not the best but the only available (thanks Martina):