The cool thing about them was that you could pick out a stone or stones, then browse a huge selection of mountings. Once you found what you wanted you could turn the stone plus setting into jewelry. You paid the price of the gemstone, the weight of the gold used in the setting, and a reasonably small fee (on the order of $20, IIRC) to make your creation a reality. Most of the rings that I wear, including my silly fish-cut sapphire, came from this site. While I missed the mark on one or two designs, I'm happy with the quality of everything I got from them.
You could also just buy tons of loose gemstones from them, including lab-created sapphires dirt cheap. A few of my friends have seen me just randomly give away a sapphire or a ruby and thought I was insane for doing so, but they were always lab-created and cost almost nothing. "Do you want two dollars?" doesn't sound nearly as impressive as, "Would you like a ruby?" but they mean the same thing.
Several years ago, Thaigem changed their business model to a dumb countdown auction model, and they've sucked profoundly ever since. The auction model is gone, but they're still a pale shadow of what they used to be.
Does anyone know of a good site with a large selection of cut natural and lab-created gemstones? I'd love to find one that will turn stones into custom jewelry for you, but I'll take what I can get. There's a pair of earrings that I've wanted to have made for about ten years, and I think it's time to actually do it.