You miss the point. The only reason to buy a diamond is that it is expensive. It doesn’t matter if you could buy the same thing cheaper. It doesn’t matter that there are better gems that are cheaper. You buy the diamond to show that you can afford it. You buy someone a diamond to prove how much they are worth to you.
Yes, you can buy a man-made diamond the same you can buy counterfeit branded clothes. That only shows that you’re tasteless and that you can’t afford the real thing.