This is just to clear up what I see as a continuing thread of semi-confusion regarding the wonderful diadem made by Garrard in 1911 for the Delhi Durbar and worn by Queen-Empress Mary on that occasion.
Yes, 5 of the Cambridge Emeralds were inverted, as it were, and stood upright at the top. And the Cullinans III and IV were set centrally. I, too, would prefer the jewel restored with these gems, but HM The Queen uses the Cambridge Emeralds interchangeably with the fantastic pearls in the Grand Duchess Vladimir Tiara. And the Cullinans III and IV have been used for many decades now joined as a brooch and HM wears it often.
Even going back into the 18th Century, there are many precedents, especially in jewels made by the major houses for Royal Families, in designing the jewels so that they were adaptable for many purposes. Many of the pieces which Prince Albert designed for Queen Victoria were able to come apart as various other jewels and be worn separately. Queen Mary originally designed the Lover's Knot Tiara with upstanding pearls completely around the jewel --- the pearls had been a wedding gift. But she wore it many times both ways, according to the photographic evidence. Once, and I forget to what occasion or ceremony (it may have been to the wedding of Wilhelm II's daughter in 1913), Queen Mary even had the Cullinan's I and II taken out of the Scepter and Imperial State Crown, respectively, and wore THEM pinned to her bodice as a brooch, with Cullinan's III and IV also worn on the same occasion! Dear Queen Mary, sadly we'll never see another like her.
Sorry to be so long-winded, but the subject fascinates me and the questions posed in this forum are so interesting! The Crown Jewels of the United Kingdom are a collection that has been formed relatively recently, but the jewelery that forms the private collection of HM The Queen is so vast that it may be hard to imagine. There are jewels we have never heard of, gifts given by the Lords of India and the Empire to Edward VII, George V, Edward VIII and George VI during the world tours taken when they were variously Dukes of York and Princes of Wales. Who knows what unimaginable treasures were brought back --- and all private property. Queen Alexandra's collection was stupendous and we know much of it was given to her grandson, the late Duke of Windsor.
Another spectacular collection of jewelry is used by the Swedish Royal Family and primarily owned by The Bernadotte Foundation, including the fabulous Leuchtenburg Sapphire Parure and SO much else.
But if I could go back in time and see any one collection of jewels, it would be the treasures of the Youssupovs. The legendary photograph of the cache found in the safe that had been bricked up in the Youssupov palace, and found several years after the revolution, makes my mouth water! The mounds of loose stones and marvelous treasure are so amazing! Also, I suppose I wouldn't say no to walking through the Grand Duchess Vladimir's bedroom with a glass of champagne, and seeing all HER fabulousness! Sigh. Those were the days...