I visited Chanthaburi gem mart last year during my vacations on the way from cambodia to bangkok. The gem mart is a corner in the city center. Not that big as I expected. May be 20 m x 20 m as far as I remember. I took some photos, beside others a dealer with 3 excellent big tanzanites. Much too expensive for me. I found other stones, trying to negotiate and to buy 2 … 3 interesting sapphires. They showed me different parcels of gemstones. After choosing some stones they said, that they only sell whole parcels. OK this was disappointing. But along the street (gem-street – some 100 meters in total) you find jewellers and dealers who sell directly at the street. There you can get stones for a reasonable prize. Knowing a bit about gemstones and compare helps. There are also some dealers near the market place (there is also the shopping centre). Seems a bit bigger than the gem mart, but fewer dealers. If you take some time, if you speak with the people and if you do not buy at the first day you may get stones for good prize. And you can also find some sellers in the old quarter along the river. The doors are open, they sit there and cut stones or sort sapphires. There you can ask. But they have mostly only small sapphires and some other stones. The most important thing you need is a good loupe (of course). And almost all dealers in the street state, that the sapphire – or certain sapphires – are not treated -> do not trust them! If you look inside you can often see evidence for treatment or at least no positve sign (unbroken silk for instance) that the stones are not treated.
And than you find a gemstone center (cannot remember the name exactly – there is a figurine with miners in front of the building, maybe 2 km or more out of the city center and normaly not to find without help). Here they sell very good stones but for very high prices.
So far about my personal impressions from 2 days in Chanthaburi.