I have this photo of Princess Margaret wearing a diamond circular Art Deco brooch.
I also think the Princess wore the same brooch pinned on her hat in that one :
I think it is safe to say that its the same brooch worn by Margaret on this earlier photo :
Regarding Queen Alexandra, the shape of the elements are different :
On the photo below, we can count 4 buttons at front, 4 at the side and we can assume there must be 4 more on the other side.... I doubt the Queen would have 12 identical big diamond brooches never seen before or since...
In my opinion, these are paste buttons decorating her long skirt.
What do you think ?