Kate's fiance Jamie Hince is said to have spent ages trying to find an original diamond ring worn by Zelda, the wife of writer F. Scott Fitzgerald, who is famous for her hell-raising lifestyle, but eventually had to have a copy made.
The 37-year-old was spotted wearing the band at a London concert this week.
A source told British newspaper The Sun: 'Jamie went to great lengths to bowl Kate over with the ring. It couldn't be any old sparkler, and he knows how fascinated she is by Zelda.'
It has also been reported that Moss could get hitched in Paris where the Fitzgeralds lived.
The source added: 'The big 'kapow' moment between them happened in Paris. They see it as their place.
'She knows Zelda spent a lot of time there and, in the spirit of the ring, Kate is talking about having the whole wedding themed around the 1920s. She wants a vintage gown and that will avoid offending anyone by picking one particular designer.'