what is stopping someone from building a high-pressure, high-temperature system and dumping graphite in and making millions of diamonds?
What is stopping someone from making these HPHT machines and making tons of money?
No one, and it is done. First of all, synthetic diamonds have been out on the market for quite some time now and it's a well established business. You've heard of diamond saws, diamond drills, diamond polishing paste and all kinds of other diamond products. So people make them, sell them, and make money.
It's not even that hard: In our own lab we got a machine that can make diamonds (among other HPHT) stuff. Someone once even put some peanut butter and made diamonds out of it (micrometre sized, though). Here's a picture of the machine (called a piston cylinder):
Here's a picture of how this might look:
Note that this one is not from our lab, but it's a really nice picture of how this stuff looks when you make it in such machines. This one's from this paper. The diamond crystals are about 30 μm across. Not something you'd put on a ring.
The big question is how do you make gemstone quality diamonds. Chemical vapour deposition seems like the most successful method. There are other methods, but it's pretty easy to distinguish them from natural diamonds (example). It is still quite hard to do it: it's not something anyone can just do in their backyard. With technology it becomes easier though. Here comes the question of regulation. Gemesis, one of the companies that makes them, engraves a serial number using laser on each synthetic gem-quality diamond they make. But what about some evil diamond makers who want to simulate natural stones? As with any other synthetic material, there are imperfections that are different from imperfections found in natural stones. Moreover, each mine or region where diamonds come from has a different fingerprint (they use that to trace blood diamonds). There are methods to analyse the diamonds to detect this. UV and IR spectroscopy, laser ablation, isotope analysis, cathodoluminescence and others. Some are easier and cheaper than others.
So you can't just make up impurities to say "look it's natural", they have to be consistent with a specific diamond source you're trying to imitate. And that's - not very easy. Likely impossible with today's technology.