The theft, thought to involve around £40,000 of gems, took place on the evening of Saturday February 21, the end of the first day of the fair.
The victim had left the jewellery in bags in his room at the Premier Inn in Southmead Way, Derby, while he was at dinner. Between 6.20 and 7.30pm, the door to his room was forced and a quantity of high-value diamond jewellery taken.
It is thought that the dealer was followed to the hotel from the fair and police are appealing for information about anyone seen acting suspiciously around the hotel or the fair. Witnesses have reported a man sitting in a Mercedes car parked in a disabled bay outside the hotel around the time of the burglary.
Contact Pear Tree CID on 0345 1233333 or Crimestoppers on 0800 555111.