The 32-year-old had asked the stall owner for a close inspection of the diamond on the opening day of Facets Sri Lanka, an annual jewellery show in the capital Colombo.
“He had taken the diamond out of a display cabinet and appeared to be appraising it and suddenly put it in his mouth,” said a police officer at the scene.
“The owner saw this and alerted us,” he said, adding that the man was arrested and taken to Colombo National Hospital to be given laxatives.
A spokeswomen at the hospital said that an X-ray had been taken and that officers were holding him under observation.
“I believe the X-ray confirmed something that looked like a stone. The man has been taken away and he is in police custody,” Pushpa Soya said.
Photographs showed the man, dressed in a black shirt and jeans with his head bowed, being escorted from the exhibition centre by uniformed policemen.