It really is a great slogan. Especially for something that is coming from a monopoly. It’s hard to say what the actual free market price of a diamond would be if De Beers didn’t control the supply. It is also a fairly recent phenomena that dictated diamond engagement rings. (see https://www.gemsociety.org/article/are-diamonds-really-rare/ for one discussion)
Of course price is usually due to scarcity, but diamonds are constantly being mined and sold. Scarcity is a relative thing.
Chocolate diamonds are making a mink coat from a sow’s ear. Water white diamonds have always had the most value, colored diamonds are basically impure carbon crystals. However, a sufficiently convincing TV ad turns it in a desirable property.
I suspect that lab grown diamonds will soon be perfect all the time. Is a stone really more valuable because it occurred “naturally”? A “synthetic” diamond is made of exactly the same material (carbon) which has been compressed to a very high pressure. That is exactly what happens to form the diamonds we mine.
Me? I’m happy with cubic zirconia. It really is pretty and if lost or stolen I haven’t given up the price of a good used car.
So what do you think? I think if I ever married again I would be very happy with a lab grown diamond, plus nobody will have been exploited for me to have it.