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 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.
The Cambridge Cluster Emerald Brooch with pear shaped Emerald Pendant and all the other "Cambridge Emeralds" was left to Prince Frank of Teck by his mother Princess Mary Adelaide Duchess of Teck, above in the picture.
The Duchess had some of the stones made into a necklace, probably in the 1820s, which she gave to her daughter, Princess Mary Adelaide, in 1857, see above in the picture.
Princess Mary Adelaide’s Journal, 23 February 1857: ‘I read with Mama til 2 when we looked at her jewels & she generously has given me her Emerald Necklace’ (RA VIC/ADDA8/3441/202).
The Duchess retained at least 11 emerald drops for mounting on another necklace (Garrard RL33, fols 230 and 304, 4 June 1868 and 15 May 1882). Approximately 30 stones altogether.
The rest of the emeralds were bequeathed to the Princess (by then Duchess of Teck) in 1889.
On the Duchess of Teck’s death intestate in 1897, her jewellery was divided between her three sons and her daughter, the emeralds (both set and unset emeralds ) becoming the property of her second son, Prince Francis of Teck.
Prince Francis, whose wayward life had often caused his family embarrassment, died unexpectedly in October 1910, leaving all the jewels he had inherited from his mother to his mistress, the Countess of Kilmorey.
Queen Mary acquired them by purchase from Lady Kilmorey shortly thereafter.
Sources:Royal Family.Royal Collection;