Betty Prest attended the thank you reception, held by former mayor Charles Johnson and his wife Carol, at the Dolphin Centre in May.
Mrs Prest was a guest of Stockton mayor Barbara Inman.
Each guest at the reception, held by Cllr and Mrs Johnson to thank those who had supported them during their year of office, was handed a glass of Prosecco as a welcome drink.
Also included was a gem, supplied by Asquith's Jewellers, in Post House Wynd, Darlington.
All but one of the gems were low-value cubic zirconia, with just one of them a real diamond.
Guests were invited to call into the shop to have their gems tested, with the lucky diamond holder having the chance to have it set in a mount of their choice.
Mrs Prest was delighted to have her stone pronounced as the real deal and chose to have it set in a white gold ring.
She said: "I thought there was a chance it was real, so decided to pop into the jewellers to check.
"On my way there, I noticed a cake in a bakers' shop window with 'congratulations Betty' on it, which I thought could have been a good luck omen."
Sure enough, shop manager Les McGhee was able to confirm its validity.
Mrs Prest was back in was in Darlington on Monday to collect her ring from the shop.
Also on hand to admire the bauble were Cllr and Mrs Johnson and Cllr Inman.
Other guests were not left disappointed, as they were offered the opportunity to have their zirconia gems made into a silver pendant.