I think it is most probably because of size, since Mary Adelaide used to wear some of the drop stones as earrings. And i think the mistress who was left the emeralds, not only recieved far more then their market value for them, but also her silence was bought with one of the brooches. I am not sure what? Possibly one of the round ones since the Duchess of Teck wore a stomacher with two round brooches and a drop. One of which now make the brooch Her Majesty wears often.
I found this interesting when i heard it, as it was never mentioned in Field's book.