Buying from your website might burn the requisite amount of money, but it wouldn't be conformist enough.

Signalling conformity with social norms is also important to some people.

I mean it is, but you can break it trivially as an individual. There's no network effect, actually there's an anti-network effect. My partner and I were patient zero for non-diamond rings in our social sphere. I got alexandrite and she got a green sapphire. The rings are stunningly gorgeous, unique, nothing at all like you would find at at Zales or whatever and less than half the price (which is good since we needed two :P)

Our friends and family were obsessed and not six months later I got a text from a now engaged friend who wanted to know how we found them. He also opted for a green sapphire because it matched her eyes.

Non-diamond rings sell themselves, someone (and it can be you) just needs to break the seal.

Proof of commitment can have utility. And status symbols can have utility.

(Going on a tangent: compare Bitcoin's proof-of-work, which also has utility. Of course, that doesn't mean diamonds nor proof-of-work are the best bang-for-buck. For the latter, you can use proof-of-stake or avoid cryptocurrencies altogether. For the former, there are plenty of other social arrangements that avoid diamond rings.)

> It’s an irrational proof of commitment.

I think that's a bridge too far.

There's many clear reasons why people want a diamond, and even an angle like "I want to follow society's expectations" is to me far from being irrational when you're prepping yourself to build a whole life with someone else.

A couple can discuss these matters and come up with alternative paths, people are referencing other materials and I'd include not buying anything fancy and save up for the first payment of their home for instance. But brides requesting a diamond is usually far from lacking rationale.