This brooch was given to Queen Mary as a wedding gift from Tsar Nicholas II in 1893, when she married George V. (This was before any of them were on the throne). It was then inherited by Elizabeth II and is part of the royal jewel collection.
It's believed to have been made by a Russian jeweller. It consists of diamonds in a three-loop celtic knot, with a cabochon emerald in the centre.
Because both emeralds and celtic knots are associated with the Irish, Queen Camilla has taken to wearing it whenever she does an engagement in Northern Ireland. I guess it is more elegant and less cliched than wearing a shamrock brooch.