Gem scams are relatively widespread in Thailand. There have been reports of visitors buying gems for inflated prices from seemingly respectable establishments then later finding out the stones are worth a tiny fraction of the purchase price.
You should report any incidents of crime to the Thai police before leaving the country.
for more crime information while in Thailand:
http://www.fco.gov.uk/en/travel-and-living-abroad/travel-advice-by-country/asia-oceania/thailand#safetySecurity