This pendant has me perplexed, as there are photos of Queen Mary wearing her version of it in 1895 and 1897. I have also read that it was given to her by King Edward and Queen Alexandra as a wedding present, although I have not seen positive evidence to support that.
The photo of Queen Alexandra was taken circa 1913.
Neither Queen Mary or Queen Alexandra are known to have shared jewels, so I wonder if there were 2 very similar brooches/pendants.