The theft occurred when two British women entered a restaurant in the luxury southern resort of Marbella and one of them left her handbag on the floor by her chair, police said in a statement.
"Two well-dressed men came in, one sitting at the bar and the other next to the woman," it said.
When the men left, the woman discovered that her handbag, containing 2,000 euros and 400 pounds sterling, a mobile phone, a pendant with a diamond worth 12,000 euros ($16,500) and other valuables, had disappeared.
Hours later, police stopped a car at a routine checkpoint and found the four occupants had criminal records.
Inside the vehicle they also discovered a handbag as well as valuables and cash, which they later identified as belonging to the British woman.
All that was missing was the diamond.
"During the operation, officers noticed one of the men putting his hand to his mouth," police said.
"This gesture and the fact that they had found the pendant without the diamond made the police think he may have swallowed it.
"To find the stone, those arrested were taken to a medical center where they underwent X-rays, and the diamond was located inside the stomach of one of them, who admitted swallowing it."(*)