Until Queen Alexandra's death in 1925, the large marquise cut stone, the Cullinan VI was in her possession.
Therefore, between 1910, when Queen Mary was presented with all the stones, and 1925, Queen Mary used the Cullinan VII as both the pendant for the Delhi Durbar Necklace, and also as a pendant from the Cullinan VIII brooch.
When Queen Mary inherited the Cullinan VI stone from Queen Alexandra in 1925, she used this larger stone as the pendant from the Cullinan VIII brooch. And she used the Cullinan VII as the pendant from the Delhi Durbar Necklace.
From 1925, Queen Mary no longer needed to swap the two marquise stones over whenever she wore different pieces. This is still how the stones are used today.
Queen Mary was also known to wear the Cullinan III stone as the pendant from the Delhi Durbar necklace occasionally.
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