I agree with OmerH: that the dark coloured stones on Queen Alexandra's coronation gown are most likely to be emeralds.

Except for Queen Victoria's emerald earrings, all the other stones appear to be cabochons, and none of them are large enough to be the sapphire given to Princess Diana.







Re QEII's sapphire parure

I read somewhere (I think perhaps Leslie Field) that King George VI purchased the suite from Garrard for QEII's wedding gift, and crucially these are faceted stones.



Re the Kent sapphire parure.

This was bequeathed to Queen Mary by her aunt, the Grand Duchess of Mecklenburg Strelitz, a sister of Princess Mary Adelaide, Duchess of Teck, Queen Mary's mother.

The parure had originally belonged to their mother, Augusta of Hesse Kassel, Duchess of Cambridge. It was given to Augusta of Mecklenburg Strelitz.

Princess Augusta, Grand Duchess of Mecklenburg Strelitz wearing the tiara.