An exhibitor at the fair reported the alleged theft on Thursday, the first day of the five-day fair.
Based on CCTV footage, police quickly obtained arrest warrants and alerted immigration police.
The two suspects were nabbed while preparing to board a flight at Suvarnabhumi Airport on Thursday. They denied any wrongdoing.
“They asked to see a diamond piece in our show cabinet,” Mantana Wiyakaew, a sales assistant at the exhibition, told police.
“After discussing the price for a while, they returned the ‘diamond’ piece to me. I checked the diamond and found that the returned piece is a fake.”
The sales assistant was able to identify the suspects but officials could not find the missing diamond.