Identified as “Jess”, the woman said she was sorry “for being such an American a**hole” and for behaving in an “inconsiderate and disrespectful” way.
The chunk of marble, believed to have possibly come from the Roman Forum, was sent to the Museo Nazionale Romano from Atlanta, Georgia.
“I would like to return this rock to its rightful place," read the note that accompanied it.
"Please forgive me for being such an American a**hole. I took something that was not mine to take.”
In addition to taking the marble while visiting the Italian capital in 2017, Jess had also written a message on it in black marker pen for her boyfriend.
The words “to Sam, love Jess, Rome” were still visible, despite her attempts to thoroughly clean the stone.
“I feel terrible for not only having taken this item from its rightful place, but having written on it as well,” said the letter.
“It was a big mistake on my part and only now, as an adult, do I realise just how thoughtless and despicable it was.”
The director of the museum, Stephane Verger, told the Il Messagero newspaper that he appreciated Jess’s decision to return the rock and said her letter was “quite moving”.
“From its tone we imagine it was a young woman,” he said. “In 2017 she must have come to Rome, and taken this fragment of marble in order to give it to her boyfriend.
“It made an impact on me precisely because she is young – she understood that she had made a mistake.”
He suggested that perhaps the events of 2020 and the worldwide pandemic had given her time to reflect on her actions.
“The fact is that three years after the theft, she returned it – it’s a very important symbolic gesture.”