Many thanks 992234177. I agree that Lady Mount Stephen's diamond and pearl tiara is also reminiscent of the Girls of Great Britain and Ireland tiara; yet it too is different.
Lady Mount Stephen's tiara which was auctioned for charity as directed by her will.
Comparison of tiaras worn by Rachel, Countess of Dudley and Queen Mary.
I have no doubt that there are other tiaras which will similarly call to mind the Girls of Great Britain and Ireland tiara.
I don't know which tiara was created first. The style was obviously favoured by ladies in the UK, as all the ones I know of are variations on a theme.
Rachel, Countess of Dudley was married in September 1891, but this tiara was not among her wedding gifts.
Wedding gifts of Rachel, Countess of Dudley.
Queen Mary married, as we know, in 1893, while Lady Mount Stephen married in 1897.
Perhaps the style for Queen Mary's tiara was copied from someone else's tiara, or perhaps from the one owned by Rachel, Countess of Dudley!