The bracelet has a double row of cultured pearls bordering a line of circular-cut diamonds.
It is around her left arm as she poses in a white tulle embroidered dress.
Beaton recalled in his diary: “She looked very pretty and wore quite a lot of make-up. And the eyes are of a piercing blue – catlike and fierce and so very pristine and youthful.
“She had been up till 5.30 the night before (she likes the anonymity of the 400 nightclub) and towards the end of the two hour sitting, started to wilt.”
Margaret, who married Antony Armstrong-Jones in 1960, died in 2002 aged 71. The bracelet is being sold by a private collector with London-based auctioneer Dix Noonan Webb.
The sale is on September 14.
Source: EXPRESS CO UK