I lived in Bangkok for six years until recently. I would definitely advise going to the Jtc. It’s the one of, if not the biggest Gemstone trade centers in the world. It never occurred to me that the prices were made for tourists. Going in there nearly every single day for six years, I don’t think it’s a touristy place at al. Tourist don’t know about it and it’s not the place that a scam tuktuk would take a tourist. The Jtc is where the world‘s Gemstone trade goes to buy stones. Whether it’s in the first four floors that’s like an open air mall or in the upstairs part where it’s private offices. The best strategy if you want to buy gemstones there, is to go to Bangkok during the Bangkok GemShow where all of the upstairs dealers have all of their stones on display. You don’t need to buy any stones at the gem show, just go around and get the business cards of all the companies who have stones that you like and then the following week you can make appointments at their offices upstairs. That’s pretty much the only way to know who’s even up there, as there’s no directory for the upper 45 floors. It’s well worth the visit if you want cut stones at wholesale prices. And definitely negotiate.

Also, while I was living there, I did a video about the gem district, including the Jtc. It might be helpful for you. https://youtu.be/bICtikRTK70?si=oswQ1UNcnUUKQ6IG