As with most jewellery of the 19th century, this piece bears no signature but the 3rd Duke of Northumberland is known to have employed Rundell, Bridge & Rundell who were the jewellers to the Crown. Documents in Alnwick Castle relate to work being undertaken by the jewellers in 1824-6 and 1828-9. An account dated 17 March 1829 states "Setting a large Engraved Emerald with Her Grace's Brilliants in a Sevigne Brooch and furnishing 2 brilliants - £80".
The one coloured gem-stone that is today most associated with India is the emerald with the first examples dating back to the Mughal period. The present emerald is exceptional because of its transparency, purity and depth of colour and one can easily understand how these stones came to bewitch the great Mughal Emperors like Jahangir (1605-1627) or Shah-Jahan (1627-1659).
Ironically the best emeralds of the period were not Indian at all but originated in Colombia. The first Colombian emerald to be discovered by the Spanish was in 1537 and within thirty years they had found the highly productive Muzo and Chivor mines. The emeralds found their way to India via the Portuguese who had settled in Goa in the early 16th century and had extensive trade links with Spain. The Mughal Empire was founded by Babur in 1527 and by the latter half of the century the Portuguese were wanting trading concessions and so introduced jewels and emeralds to the Mughal Emperors. By the 17th century the master carvers in Jaipur were producing wonderful carved emeralds with floral motifs that were derived from the 17th century architecture, as well as to bring out the full colour and beauty of the stone.
The most popular way of wearing a large emerald such as this was to attach it to a string of pearls as a pendant. The present emerald does have a surmount, now disguised by the diamond mount, that could well have been used for this purpose.