[Spivak]

It's all kind of arbitrary, some things catch on. Rings as signs of commitment and everlasting love go as far back as ancient Rome. Precious metals like gold for the actual ring symbolize the same thing because they don't rust. And DaBeers had the right messaging at the right time to popularize diamond, a white stone that matches a wedding dress, is clean and pure meshing well with Christianity in the US, and was already mythologized as the hardest and unbreakable.

It's the same symbolism for why it's popular for guys' wedding rings to be made out of super strong, super hard metals.

People give DaBeers too much credit for what was an extremely natural extension to the engagement ring. Advertising only gets your foot in the door, it does have to be a reasonably good idea for it to take on a life of its own.