It's marketing plus perception of how expensive it is. Most of diamond purchases are for engagement rings. Nobody wants to appear cheap. The expense and rarity is the point.

That’s a silly way to look at it. Surely people can realize that something made through natural processes has more of an appeal than something made in a factory?

That said, most of the gems I’ve purchased in my life have been lab grown.

It’s kind of surprising that diamonds still have appeal as jewelry at all given the rise of lab-grown. I always assumed that people liked them because they were rare and expensive.

In final jewelry no. This is why brands like Tiffany are in panic and pivoting to mechanical-watch-like branding, where only inflated price matters.

De Beers sells devices that can distinguish between naturals and CVD synthetics. They're not cheap, but less than ~$80K, IIRC. They do a pretty good job, I've heard >90% success in identifying CVD stones.

With close microscopic examination of inclusions and defects, yes you probably can. There are also spectroscopic differences. In general looking at finished jewelry, no, not really.

There are some applications, such as IR optics, where natural diamonds aren't pure enough.

DeBeers has been working on systems to do that.

My wife and I got our engagement and wedding rings from Krikawa[1]. The stones on her ring were synthetic, extremely affordable given the size, and visually flawless.

(Not quite as related, but the process was also really easy; we were able to communicate everything over email, get sizing kits mailed to us rather than having to go in person, and they sent us visual mock-ups and procedurally generated 3D videos of what the results would look like, which was helpful because the rings my wife picked out had been temporarily delisted as they found an issue with it that they wanted to fix, so they went ahead and figured it out so they could make them for us without us having to wait for them to appear on the site again).

[1]: https://www.krikawa.com/

Synthetic gem-quality diamonds are old news by now. Since the 2010s, you have been able to make essentially flawless diamonds by vacuum deposition, of higher quality than anything found in nature, weighing up to a hundred karats or so.

They’re also vastly more ethically produced than most natural diamonds, and don’t have prices inflated by the artificial scarcity imposed by the De Beers monopoly.

https://en.wikipedia.org/wiki/Synthetic_diamond

Try somewhere like Blue Nile [1]. The price of natural diamonds increases exponential once you go over 1ct and the size becomes rarer.

Here's a quick comparison on just size, clarity and the visibility of inclusions.

A 1ct very good quality stone, E-F, VVS2 or better, no fluorescence - you're looking at a 88% reduction in cost ($700 vs $6,000).

Jump to 2ct and it's $2,300 vs $32,000.

At 3ct, the lab grown is still only $4,200 where the natural at that size starts at $82,000.

[1] https://www.bluenile.com/diamonds

How large do you want them?

> https://item.taobao.com/item.htm?id=828627288905&skuId=55567...

This ring goes up to 5ct, and some other listings are at 7ct.

A 3ct example at the link, which would easily be a >$20k ring under other circumstances and isn't too comically large, will run you about $1500.

There are many, many others.

https://old.reddit.com/r/labdiamond/

James Allen, Brilliant Earth, Jared, Blue Nile, all of these vendors sell lab created diamonds openly and let you compare side by side. A diamond that’s $10k natural can be had for $1500 lab created without any scarcity.

As a fan of cool rocks and gems, and putting aside price and societal influences, diamonds are cool!

Especially compared to hard plastic “costume jewelry” (which I think you’re referring to), gems are hard, don’t scratch as easily as hard plastics, and have cool reflections.