The passenger lost the priceless object as they disembarked a ship.
The Cunard spokeswoman said: "It was picked up by another kind passenger who couldn't find a member of staff at the time, so the lady took it home with her and posted it back to our customer service team.
"Our customer service team had already been alerted to the missing item by the guest who had lost the ring, and our team immediately recognised it from the discussions they had with the owner.
"The owner was reduced to tears when informed it had been found. Once returned, they visited the jewellers where it was valued at £1,500 ($3181), but it was the sentimental value of the ring that made it priceless to our guest."
On another occasion, a Cunard Line employee found a solid gold Rolex watch in a stateroom.
The spokeswoman said: 'The guests reported that they had left it on board and after corresponding with the ship the customer service team were able to confirm that it had been safely found.
'Once the ship had arrived in Southampton the guests decided to drive from Exeter to collect the watch from the offices due to the value and sentimental value of the item.
"The guests were so relieved, and grateful to be reunited with the watch, the couple in question presented the customer service team with a beautiful bouquet of flowers as a thank you."
An employee who works for a different cruise line and requested anonymity told MailOnline her colleagues have found discarded sex toys, more than US$500 ($739) in cash in an envelope and false teeth after passengers have checked out.
Passengers have gone online and shared their amusing or shocking tales about items they forgot on a ship or the unusual belongings that didn't go home with the previous guest.
In a forum on CruiseCritic.co.uk, one user wrote: "Was having trouble closing the lower draw of the cabinet. Dug deep behind the drawer and came up with a half dozen pair of women's panties. Thankfully all clean and folded."
Another holidaymaker added: "The room steward had not cleaned under our bed for a while which became apparent when putting the suitcases there.
"There were a couple of used Depends [adult nappies] and a condom. While possible ... seems unlikely those things would have been left by the same passengers."
One user was stunned when they visited lost and found, adding: "I left my glasses behind in the theatre one evening, went back to retrieve them but they had gone.
"Checked with customer services but no one had handed them in. They funny thing was the lady on the desk brought out a whole range of glasses and sunglasses and asked if mine were amongst them. There were about 20 pairs all from that cruise and we had only been on the ship a few days."
Adam Coulter, UK editor of CruiseCritic.co.uk, told MailOnline passengers usually leave behind items such as mobile phones, digital cameras, shoes or underwear.
He added: "As with hotels and resorts on land, it's easy to leave items on board at the end of your cruise.
"Most lines will try and help recover missing items, but there's no absolute guarantee.
"Allow extra time to pack, and always do a final thorough cabin search before you disembark, including drawers, under the bed, and the back corners of the safe, to avoid that last-minute packing dash."
Passengers who have left an item on a cruise ship usually have a limited amount of time to claim it before it is disposed of.
Carnival Cruise Line, for example, has a policy that states items forwarded to the lost and found department will be kept for 30 days.
If a lost item has been recovered and the passenger wants it returned it will be shipped at their own expense.
But not all items will be returned.
Some cruise lines refuse to ship electronic cigarettes due to concerns over leaks or damage, medical items, food or drink, items that have been in close bodily contact or duty-free purchases.
