A "family heirloom" has been lost after its aristocrat owner wore the emerald and diamond pendant on a shopping trip to a Morrisons supermarket.
Lady Somerleyton returned from buying groceries at the store in Pakefield near her Somerleyton Estate home in Suffolk on 9 October when she noticed the Art Deco style jewellery was gone.
She had also visited the village of Henstead on the same day.
A "devastated" Lord Somerleyton has offered an undisclosed cash reward.
The pendant was attached to a 46cm (18in) chain, the family said.
"My wife and I are devastated to have lost the pendant which is a family heirloom, and therefore I have decided to offer a cash reward for its safe return, should someone find it," Lord Somerleyton said in a statement.
A Morrisons spokesman said the lost pendant had been reported to staff who were "keeping their eyes peeled".
"We have our fingers crossed that it turns up," he added.
Suffolk Police also confirmed it had been informed of the lost item.
Lord and Lady Somerleyton's family home Somerleyton Hall, near Lowestoft lies within a 5,000-acre (2,023 hectare) estate.
The value of the pendant and the value of the reward have not been disclosed by the Somerleyton family.