"It has been lost for a long, long time because it was covered in sediment. It belongs to a couple who got married on the 17-02-1979.
"CAN YOU CAN SHARE THIS, please? For me it is very important to give it back. Many thanks."
Ms Cuesta added in a video posted by the BBC: "When I looked at it in more detail I realised that it had a name on and date and that it was a wedding ring.
"It was then we felt a little bit sad because you think of the poor person that had lost it."
The post was shared by more than 80,000 Facebook users and was eventually spotted by a stunned Agustin Aliaga and his wife Juani Sanchez.
"[My wife] said 'yes, yes, go and look on Facebook'," Mr Aliaga explained.
"I went to look and there you go, it was my ring with the date of our wedding."
Mr Aliaga said he lost it while swimming several months after getting married. Ms Cuesta gave the ring back to the grateful pair at their bar in a village near the northeastern city of Zaragoza this week.
Still, at least he didn't accidentally throw the ring out with the rubbish.
Meanwhile, a woman who had a crush on the Waterstones Twitter guy has revealed how she ended up marrying him.
Is this dress too short for a wedding? This woman's outfit has stirred an online debate.