Gem scams are so common in Bangkok that TAT has published a brochure about it and there are several websites on the subject, including the very informative w2bangkok.com/2bangkok-scams-sapphire.html, which describes typical scams in detail. Never buy anything through a tout or from any shop recommended by a “government official”/“student”/“businessperson”/tuk-tuk driver who just happens to engage you in conversation on the street, and note that there are no government jewellery shops, despite any information you may be given to the contrary, and no special government promotions or sales on gems.
The basic scam is to charge a lot more than what the gem is worth based on its carat weight – at the very least, get it tested on the spot, ask for a written guarantee and receipt. Don’t even consider buying gems in bulk to sell at a supposedly vast profit elsewhere: many a gullible traveller has invested thousands of dollars on a handful of worthless multicoloured stones, believing the vendor’s reassurance that the goods will fetch at least a hundred percent more when resold at home.
If you’re determined to buy precious stones, check that the shop is a member of the Thai Gem and Jewelry Traders Association, by visiting their website which has a directory of members (wthaigemjewelry.or.th). To be doubly sure, you may want to seek out shops that also belong to the TGJTA’s Jewel Fest Club (wjewelfest.com), which guarantees quality and will offer refunds; see their website for a directory of members.