If you are based in either the US, UK, or Germany, you might take a look at Gems TV. It's all hand made in Thailand, shipped and stocked to Gems TV warehouses and sold via the TV channels or internet in Europe/USA. You get a guarantee of authenticity and money back guarantee if you don't like what you buy. Good prices too. I've bought from them and have been most impressed.
Gems TV

In Bangkok I found a number of what appear to be reputable jewelers in Silom (not the Sala Daeng end, the end towards the Oriental Hotel/Chao Praya River) but let the buyer beware.