Since I'm not an expert at evaluating gemstones, and don't have one to assist me, I wouldn't trust any gems on sale in Thailand. Call me a cynic.

If I were looking to just buy something in gold, that's different. I would trust a number of larger, long-established gold shops, but there's one thing you need to know: if you buy jewelry (as opposed to bullion) you won't be able to sell it back to any shop except the one that sold it to you, and even then they will come up with fees or other tricks to ensure you don't get the full buy-back value. This despite the "guarantee" on the receipt.