British Vogue’s jewellery and watch director, Rachel Garrahan, said: “What could be more romantic than a toi et moi ring, where two stones meet to symbolise the union of two lovers?
“Dating back centuries, the style became famous in 1796, when Napoleon Bonaparte proposed to Josephine de Beauharnais with one.”
Ms Garrahan added that although the rings traditionally “feature two different stones in matching sizes and cuts”, contrasting designs are also popular now.
At the time of their engagement in 1967, Margrethe was still a Princess and Henri de Laborde de Monpezat, later Prince Henrik, was a French diplomat.
Henrik purchased the ring from renowned French jewellery house, Van Cleef & Arpels.