The militaries of the world invented it, it's business that bought it. Much of this diamond/marriage symbolism stems back to the late 1940s post WWII with DaBeer's engagement ring ad campaigns.

WWII involved a lot of R&D in psyops and effects of propoganda. Sure, these strategies always existed but it became part of scientific research, was refined and weaponized to manipulate perceptions of people using non-kinetic approaches to try and avoid or minimize kinetic warfare. After the war in the mid 40s ended, where do you think all that expertise in propoganda from military went? Business marketing and advertising sprouted from much of this expertise. Marketing and advertising always existed before then but there was a dramatic shift in how things were sold creating armies of refined snake oil salesmen.

In the late 40s, DaBeers ran a massive ad campaign employing such propoganda that shifted culture into associating diamond rings with marriage. There had been dowries and other exchanges of wealth and power in marriages before (it's often been a basis for marriage) that but DaBeers managed to shift that in culture in the west to the diamond ring. It's now so deeply ingrained in culture and people's perceptions that it can make otherwise rational people irrational.

Do not ever underestimate the power of propoganda in its various forms. Pyschogical manipulation runs rampant in business marketing and these are the effects. We've culturally accepted it for a variety of reasons. I question if we should continue to accept these practices in business.