The person who took the rock from the Abbeycwmhir, said the were an avid follower of the Welsh Kings and their history, and so took the rock on a visit.
Llywellyn ap Gruffyd was buried at the Abbey and has a memorial stone in the ruins.
But the person, who described themselves as an Australian fan, said since they took the rock, they have suffered bad luck.
The stone was posted to the Abbeycwmhir Heritage Trust cellotaped to a greetings card made in China with a garden scene on the front and a message scrawled across the other side.
The "fan" of Abbeycwmhir wrote: "I am so sorry for taking, borrowing, stealing this piece of the old Abbey Cwmhir.
"I have been an avid follower of the Welsh Kings and their history and so I took this rock. Ever since I have had the most awful luck as if Llewellyn himself was angry with me.
"So I am sending it back. I will not leave my name or address, just a heartfelt sorry from an Australian fan."
The stone has now been placed back at the abbey.
Roger Coward of the trust said they have been trying to track down who sent the stone.
He said: "We were very impressed that someone had taken the power of the Abbey stone so seriously, and that they believed that Prince Llywelyn, the last Prince of Wales, who is buried in the Abbey had a moral influence over them.
"The unknown sender is clearly contrite and more than anything doesn't want to harm the existing remains of the abbey.
"We have placed the returned stone next to the Abbey ruins in the Exhibition Room at Home Farm provided by CADW. We work alongside the owners of the land on which the ruins stand, Mr Mel Hamer and Mrs Anita Hamer.
He added: "We have tried to use our contact in Australia to find the owner. In fact, the owner of the Abbey Estate between 1824 and 1837, Thomas Wilson Esq went bankrupt and emigrated to Australia and became the second Mayor of Adelaide.
"When I was editing the book Abbeycwmhir History Homes and People I had contact with a curator in the State Library of South Australia but he could find no-one."
The trust were unaware of when the stone, which measures 2cm by 1cm was taken, but hope the sender begins to get more luck now they have returned it.
Mr Coward said: "We do hope that returning the stone has brought better luck to the sender.
"In fact we wish him or her the very best in all possible ways and send our thanks for the stone.
"To be honest we had no idea the stone had been taken so it came our of the blue."