Jiang Xulian, 30, confessed late Friday night (September 11) after an X-ray scan showed a diamond-like object in her large intestine, said police.
The woman was later given laxatives before she and another Chinese man, identified later as Hae Ying, 30, were held in police custody for legal action.
The duo were earlier detained on Thursday night (September 10) at Suvarnabhumi airport on on suspicion of stealing a six-carat diamond at the Bangkok Gems and Jewellery Fair at Impact Muang Thong Thani in Nonthaburi.
Pak Kret police were informed by an exhibitor at the fair that a couple who appeared to be Chinese nationals had switched a fake diamond for a genuine six-carat stone worth around B10 million on the first day of the fair.
Video footage from the CCTV at Challenger building, where the fair is being held, showed the two Chinese at the booth.
Immigration police later detained a Chinese couple whose appearance matched the video images. They initially kept denying the accusation, claiming that they were just tourists visiting Thailand and about to return home.
However, police did not find the stolen diamond with them at the time. The duo were later asked to undergo an X-ray scan, which showed the stolen diamond in the woman's large intestine.