What is Gem Scam?

Legal Definition
The gem scam is a confidence trick performed in Bangkok, Thailand as well as other cities in the country. It is one of the most pervasive scams in Thailand. Most of the shops are gold or jewelry shops. It has been alleged that this scam has been operating for the past 20 years, sometimes in the same premises. It has been suggested that individuals in the Royal Thai Police and even politicians protect this scam. The scam was demonstrated on an episode of the BBC series The Real Hustle and in a Scam City episode in Bangkok.
-- Wikipedia