That is Queen Alexandra's Dagmar necklace. It is a historical piece. Queen Elizabeth has removed two large pear shaped pearls from it (they should be inside the two loupes) and a large cross from the middle.
Leslie Field writes:
"In 1863 Princess Alexandra of Schleswig-Holstein-Sonderburg-Glückburg's father was the elected heir to the childless King Frederick VII of Denmark. For her marriage to the Prince of Wales that same year, the King had the crown jeweller in Copenhagen, Jules Didrichsen, design a necklace in the Byzantine style. It had 118 pearls and 2000 diamonds. Festoons connecting gold medallions, with a large diamond in the middle of each, surroud a centerpiece of diamond set scrollwork. The two large pear-shaped pearls on either side were so valueable they had been exhibited at the Great Exhibition at the Crystal Palace in 1851. --
--Hangin on a gold loop from the cenrrepiece is a cloisonné enamel facsimile of the eleventh-century gold Dagmar Cross, in which was set a fragment reputed to belong to the True Cross and a piece of silk taken from the grave of King Canute. Queen Dagmar had been the benevolent and much loved wife of King Waldemar the Victorious. When she died in 1212, she was buried with this pectoral cross upon her breast, and when her tomb was opened centuries later the cross was removed as a precious relic and put on display in the Museum of Northern Antiquities in Copenhagen. It became a tradition that Danish princesses were given a copy of the cross when they married..."
... couldn't find a picture of the necklace but found Queen Alexandra as we all remember her... There is some HEAVY jewellry