Steph Gardner's partner Rob Clarke proposed in November with the platinum ring gifted to him by his grandmother.
The ring went missing on Friday morning and she believes it came off outside Cummins, where she works.
Steph told The Gusher: "I'm heartbroken.
"I got out of my car and walked into Cummins. I think it came off outside the building as it was freezing and could've been slightly loose."
She walked out of her workplace and back to her car before visiting her parents in Borough Hill.
The bride-to-be said: "I left my parents at about 3.45pm and parked in the Aldi car park. I then went into the store and got a few bits before realising my ring wasn’t on my finger. I completely panicked and ditched my little basket and ran back to my car."
Since losing the ring, Steph has retraced her steps twice and even used a metal detector in the hope of finding the treasured item.
"The ring is from Rob's nan," Steph added.
"His grandad gave it to her and she gave it to Rob to give to me. He had it specially made."
"I've nearly lost all hope. Please, please, if anyone sees or hears of anything then let me know. It's so precious to me and has huge sentimental value to us."
A reward is being offered to the person who finds and safely returns the ring. If you find it, telephone 07803 505995 or email [email protected]