Queen Alexandra wore it to the Opera evening for the Danish State Visit in 1914.
From the sketch it appears that Queen Mary is wearing the koh-i-noor. I assume that she was wearing her own crown, but Nellie will no doubt know when HM had the arches made removable and whether this is QM's crown or Queen Victoria's Regal circlet.
Below is a description from The Times of the jewels worn by the British Queens and the Queen of Denmark.
Does anyone have ideas about the crown or tiara which Queen Alexandrine wore?