University metalworkers quickly replaced the piece Friday, repairing damage inflicted on the floral or leaf filigree.
The damage probably was accidental, not deliberate vandalism, said Dexter Adams, director of grounds with the UGA Physical Plant.
"I think it was inadvertent, because the piece was left on-site," Adams said Friday. "Apparently, it sat there for about a day before anybody noticed it wasn't symmetrical."
Adams learned of the damage from Josh Delaney, UGA Student Government Association president, who notified UGA officials about the break Tuesday in an e-mail.
The broken piece was sitting on a ledge in the top part of the Arch.
A friend saw the damage happen, Delaney said Friday. Students were climbing on the Arch sometime late Saturday night or early Sunday morning - probably celebrating graduation, he said.
Graduating students often pose for photos at the Arch, sometimes climbing it part way for their poses.
A happy graduate could have grabbed the decorative piece during a group photo and accidentally broke it, Adams said.
Metal pins that held the broken piece in place snapped, he said.
The Arch, which workers stripped and repainted last month, seldom gets damaged or vandalized. In fact, it's the first time Adams can remember damage to the Arch in his 27 years in Athens, he said.
"I think everybody's got a connection to that Arch," he said.
University officials don't know exactly when the Arch was erected on UGA's historic North Campus, though it was in place at least by 1875.
University records show officials had laid plans for the Arch as far back as the 1850s, however, said Janine Duncan, campus planning coordinator for the physical plant.
Morris News Service writer Lee Shearer contributed to this report.