"A female foreigner left an emerald ring valued at HK$12 million in the washroom on the fourth floor of the Four Seasons Hotel in Hong Kong," a police spokeswoman said.
"A few minutes later when she returned to look for the ring, it was gone."
The South China Morning Post quoted an unidentified employee of Chopard as saying the ring belonged to Caroline Scheufele, the high-end jewellery maker's co-president who was in Hong Kong to celebrate the opening of a new store.
Police would not confirm the identity of the ring's owner, but announced that a reward of HK$500,000 had been offered by the victim for the return of the ring.
The ring was lost on September 8th around 11.30pm. Its disappearance has been classified as theft, police said.
It features a 16.83 carat emerald sitting between two diamonds of 1.15 and 1.29 carats, with another 674 small diamonds around the band.
Chopard Hong Kong and Four Seasons Hotel did not immediately respond to requests for comment.
Hong Kong is also the centre of a dispute over the ownership of a 16-carat yellow diamond reportedly stolen from ultra high-end jeweller Graff Diamonds.
A Hong Kong pawnshop which bought the ring is fighting to keep the gem after the London jewellers claimed it was stolen from them five years ago.