It is part of the Cambridge Emerald Collection:
The story begins in 1818, when King George III's seventh and favorite son, Adolphus, Duke of Cambridge, married Princess Augusta of Hesse...While they were visiting Frankfurt, a State lottery(auction!) was held in aid of charity; the Duchess bought some tickets and won a small box containing some forty graduated cabochon emeralds. Back in England she used some of them to make a pair of drop earrings and a necklace with five pendant stones.
Augusta of Cambridge, had left the emeralds, approximately 30 stones altogether, to her daughter Princess Mary Adelaide, Duchess of Teck.
The Cambridge Cluster Emerald Brooch with pear shaped Emerald Pendant and all the other "Cambridge Emeralds" was left to Prince Frank of Teck.
Princess Mary Adelaide had died intestate in 1897 and around that time Prince Frank of Teck, the brother of Queen Mary acquired the Cambridge emeralds.
Prince Frank gave these jewels to his mistress, Ellen Constance Kilmorey, the none-too faithful wife of an Irish Earl.
Eleanor (or Ellen) Constance (before 1866–1920), wife of the 3rd Earl of Kilmorey, daughter of Edward Holmes Baldock, MP for Shrewsbury. She had previously been one of the mistresses of King Edward VII (Magnus 1964, p. 299).
Later, the gems were quietly bought back from the Countess by Queen Mary to the sum of £10,000, the equivalent of more than £1.2million today.
Above Queen Mary with the Cambridge Emeralds in the earlier setting, and as pendant on the necklace, which she wore on the investure of her son Edward Prince of Wales.
Sources:Royal Family.Royal Collection;