It's based on supply and demand.
In 2012, when I was shopping for an engagement ring, a natural 2ct diamond cost $250,000. (I bought a 2ct moissanite for much, much less, and my wife is very happy with it.)
When I looked in fall 2023, a natural 2ct diamond cost $20,000. That's a loss of over 90% of value, not counting inflation! (Now the supply of diamonds is much higher, and the demand for natural diamonds is much lower.)
I suspect that natural diamonds will be sold for a 40-300% premium over manufactured. I also wonder if impurities will become fashionable, just to show that a specific diamond is actually natural and can't be made in a lab.
> Artificial diamonds
BTW, there is no such thing as an artificial diamond. A manufactured diamond is 100% diamond, for all intents and purposes.