Dear Sam,
Princess Diana wore on least 3 times this aquamarine ring.
She wore the ring for the first time in Italy with her aquamarine earrings and her pearl and aquamarine bracelet before her divorce in 1996.
It is now known that the stone was given by her best friend
From wikipedia :
"The ring, an emerald cut aquamarine flanked by small, solitaire diamonds and set in 24-carat yellow gold, was commissioned by the Princess from Asprey in 1997 and was used by the Princess as a replacement for her engagement ring after her divorce from the Prince of Wales in 1996.It was set on 24-carat yellow gold to match her pre-existing aquamarine bracelet which had been a gift to her from the Royal Family.The stone was given to her by Lucia Flecha de Lima who was known as the Princess' best friend.The stone that is placed on the ring had been taken out of a mine in Brazil where the Queen's aquamarine pieces are usually taken from."
Source : Pham, Jason. "Every Time Meghan Markle & Kate Middleton Have Worn Princess Diana's Jewelry". SheKnows. Retrieved 14 September 2019.
I also do think that Princess Diana was given an other "aquamarine ring" by Lucia Flecha de Lima :
Franck
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I cannot comment either on the stone. DIana did were it to both charity viewings when she old of her dresses. From memory Meghan also wore rhe ring on the way to the wedding reception.
Mike
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While I don't have any details on carat size, I do know that the Aquamarine is emerald-cut, set in 24-carat yellow gold, and that the ring was bought at Asprey, supposedly to go with an aquamarine and pearl bracelet.