Hi.

I was having difficulty to a Thai friend explaining the concept of a "con" or "scam" .. eg Bangkok Gem Scam. They closest we could get was "game" or "trick". What would be be the closest translation?

Also "con-man" or "scammer".

'lawk' หลอก or 'lawk-luang' หลอกหลวง is the most common expression for 'to cheat'; 'to scam'.

'gon-gohng' กลโกง is a trick or a scam.

In some scams, the word 'phii' ผี (normally meaning 'ghost') will also be used, as in 'khao khaay dtua phii' เขาขายตั๋วผี (he sells counterfeit tickets). Note that this 'dtua' ตั๋ว has a rising tone unlike the 'dtua' ตัว which means body (mid tone).

In your example, I would try an explanation such as

lawk hai seu phet khunnaphaab dtam. หลอกให้ซื้อเพชรคุณภาพต่ำ (trick into buying low quality gems)