On page 111 of the 1987 edition of The Queen's jewels Field says that this brooch formed part of the collection of jewels Queen Mary assembled for Princess Alice, Duchess of Gloucester's wedding gifts. Field also refers her readers to a photo of Queen Alexandra on page 59 of the book. That photo is the same one Baxter used in his post.
Initially, I thought that the two brooches were different as the one pinned to Queen Alexandra's skirt looks more solid, but after I looked at the collage Nellie posted showing the brooch as the centre of the Queen's necklace, I began to have second thoughts. I am still undecided.
The present Duchess wearing the brooch. [img][/img]
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I think this is the same brooch but it looks different as it is at a different angle [img][/img]