I like the fact that Hugh Roberts organizes his book The Queen's Diamonds by successive queens, but it is a little hard to trace the history of Queen Mary's tiaras. I wish he had grouped them together and described the many changes in one place.
Queen Mary's original 1893 wedding gift tiaras could also be worn as necklaces. This was true of the very similar gifts from the County of Surrey and Queen Victoria, and of the gifts from the Ladies of England and the Girls of Great Britain and Ireland. (Roberts says three tiaras were exhibited. Maybe that was because Queen Mary wore Queen Victoria's gift as a tiara at her wedding.)
Of these four wedding gifts, only the Girls tiara has survived, in slightly altered form, Queen Mary's urge to have new pieces made as she matured and reigns succeeded each other. This tiara was given to Princess Elizabeth at her wedding in 1947 and it has probably become her favorite tiara. No one seems to have been photographed wearing the Girls tiara as a necklace. Queen Mary did wear it once as a coronet.
The County of Surrey necklace/tiara provided the large diamonds later placed atop the Girls of Great Britain and Ireland tiara in place of the upright pearls, and part of the diamonds for the honeysuckle tiara Queen Mary had made in 1914. She gave the honeysuckle tiara to the Duchess of Gloucester in 1935.
The diamonds from Queen Victoria's necklace/tiara gift became the Fringe tiara in 1919. Perhaps because of its history, this tiara was worn by Princess Elizabeth and Princess Anne for their weddings. The Queen has now inherited Queen Mary's Fringe tiara from her mother, to whom Queen Mary gave it, and has worn it once in recent years.
Part of the diamonds from the Ladies of England tiara became the Lover's Knot tiara in 1914, along with the upright pearls from the Girls tiara. Queen Mary evidently decided upright pearls were out of style, since she eventually had them removed from the Lover's Knot tiara also. The rest of the diamonds from the Ladies of England tiara were used in the honeysuckle tiara now worn by the Duchess of Gloucester.
The tiaras Queen Mary had made or acquired later were rigid and worn only as tiaras.
The Crescents tiara that she inherited from her mother in 1897 had been assembled for the Duchess of Teck from a collection of diamond roses and crescents. This tiara, like several heirloom pieces of jewelry, has survived in the Queen's collection. It was worn at least once by Queen Elizabeth and several times by Princess Margaret. Roberts says it is loaned now to the Duchess of Cornwall, but she has not worn it.
Another early tiara that was broken up was the Boucheron loop tiara, which was made in 1902 from stones given to Queen Mary, then Princess of Wales, by deBeers in 1901. It was worn for the Coronation of King Edward VII in 1902, along with the County of Surrey gift as a necklace. See Nellie's photo at
The loop tiara was broken up and the deBeers diamonds were used again in 1911 to make the Delhi Durbar tiara. This tiara has been used with the Cambridge emeralds and two of the Cullinan diamonds, but those have all been removed. Roberts remarks that the front of the Delhi Durbar tiara has been slightly altered, but doesn't describe the alterations. Obviously, some small diamonds were removed from the top, and the space on the front where a Cullinan was worn has been filled with other diamonds. Queen Eizabeth (later QM) and the Duchess of Cornwall have each worn the Delhi Durbar tiara once.
Queen Mary acquired the Vladimir loop tiara in 1921. The only change she made was to have the Cambridge emeralds fitted as alternates to the pearl drops. Like the Fringe tiara, this one has needed to be rebuilt, but its design was not changed. The Queen frequently wears the Vladimir tiara.
Other tiaras acquired by Queen Mary have gone to different parts of her family and are not covered in the Roberts book. This is true of the Kent sapphires and other Kent tiaras, some of the Gloucester tiaras, and some of the Harewood tiaras. During the early 20th century, Queen Victoria's tiaras were scattered among her descendants. A complete history of all of those tiaras would also be very nice to have.
I hope this summary will prove useful. Please feel free to correct it and add information as necessary!
Edit: I may be wrong about Princess Margaret wearing the Teck Crescents tiara, as well as the Teck hoops necklace which she did wear frequently and left to her son. If anyone sees a picture of her in the tiara, please post.