I just love tiara-talk!
Regarding 'tiaragate' - I've seen reports from a number of experts which say they think the tiara in question is/was Queen Mary’s Sapphire Bandeau, also known as the Maria Feodorovna Sapphire Bandeau.
The central sapphire is a removable brooch. The tiara can be fitted with other central stones, including a large, carved emerald brooch surrounded by diamonds.
It's said that Queen Mary bought the tiara at auction in 1921 from Princess Nicholas of Greece, at the same time she acquired the Grand Duchess Vladimir Tiara, which was also from Princess Nicholas. (Princess Nicholas was originally Grand Duchess Elena Vladimirovna of Russia). The actual Grand Duchess Vladimir Tiara can be worn as-is or be fitted with either pearl drops or emerald drops.
Unlike the Grand Duchess Vladimir Tiara, no-one knows where or how Princess Nicholas acquired the bandeau tiara.
Maria Feodorovna (the first known owner) is the aunt-by-marriage of both Queen Mary (HMQ's grandmother) and Princess Nicholas.
It is possible that Princess Nicholas was given the tiara by Maria as a wedding present, but no-one knows for certain.
Maria Feodorovna was also the mother of Nicholas II of Russia, which goes some way towards explaining why her son, Nicholas II, and Alexandra’s son, George V, looked so much alike.
If I've attached it properly, the picture shows Queen Mary wearing the tiara with the carved emerald & diamond centre brooch as opposed to the sapphire. It looks similar to the one that Meghan chose for her wedding.
Princess Margaret wore the sapphire version in the 50's & early 60's. It's not been seen in public since then.