During that discussion I wondered if Princess Marina had been given a tiara by the Duke of Kent - possibly one for which we do not have any photographic evidence or even mention in the wedding gift lists, as one media report of the pre-wedding reception spoke of HRH wearing a tiara which was a gift of the bridegroom.
I have now found that I have another reference to Princess Marina wearing a bandeau which I cannot identify. Perhaps others might recognise the jewel from the description below. Or perhaps HRH did have other tiaras which we have not previously identified.