Unusual Late Victorian pave Pearl heart locket, modelled in 18 carat gold, with NAN in gold edged letters set with faceted rubies.
Etymonline describes nan as originating as a child's word for "female adult other than mother". This is why nanny is used as the word for a caretaker of children (since the 18th century) as well as a grandmother (since the early 20th century)
As this locket dates around 1880 the word NAN here could be short for Nancy 
Measures - 18mm x 17mm
Weighs - 5.9g
18 carat gold, pearls and rubies