As far as I know all the diamonds in Queen Mary's crown were her private property, except for the Koh-i-noor diamond which Queen Victoria made an heirloom of the Crown.

In the front of the band, Queen Mary had Cullinan IV mounted and Cullinan III was set in the cross surmounting the monde. The Koh-i-noor was set in the front cross pattee.

After Queen Elizabeth the Queen Mother became Queen the Koh-i-noor was set in her crown and Queen Mary used the heart shaped Cullinan V in its brooch setting to replace the Koh-i-noor.



As a circlet with a crystal replica of the Koh-i-noor and Cullinan IV in the band.



Cullinan III & IV in the brooch fitting.



Some of these changes can be followed in the Royal Collection site at https://www.rct.uk/collection/31704/queen-marys-crown

For the 1937 coronation (when the Koh-i-noor was given to Queen Elizabeth the Queen Mother) Queen Mary substituted the Cullinan V in its brooch setting in place of the Koh-i-noor and modified the band so that she could wear Cullinan III & IV in the brooch fitting as a corsage jewel, both of these diamonds being her personal property.

Cullinan V as a brooch


1937 coronation showing Queen Mary wearing Cullinan III & IV as a corsage jewel and Cullinan V in the circlet portion of her crown.

1937 coronation


How the Cullinan V brooch was used can be seen more clearly in this 1938 photograph.