I wonder....I have a couple of more images of this brooch.
You can see what looks like similar style of brooches being worn in on the left hand side of her dress collar. (or its the necklace below hidden among her other jewels)
This is a well known image of Alexandra's wedding jewels. Notice the two pieces below the necklace. I wonder if they are convertible brooches. They are not shown with the full set in photos.