Hello,
This still image from British Pathe clearly shows the same brooch I think.
Now, several of the small diamonds between each larger one are visible.
If Queen Elizabeth II inherited it from her Grand Mother in 1953, it's curious she tooks decades before wearing it...
I think that before aging, the Queen was not very fond of lilac \ purple \ pink color jewelry as much as Queen Mary did.
What do you think the gem can be on this one ? I guess for a pink sapphire.
Have a nice day !
J-F