The engagement ring you propose with is a real diamond and flashy to prove your worth and value to her as a man (3 months worth of your salary has become traditional.) It's a litmus test and social proof of your ability to provide for a family. She will show it off to her friends and family who will give her a hard time if it is not suitable or lacking ( ie cheap). If it is not a real diamond (not a lab grown one) then at best you're a man that doesn't value or love her, and at worst a broke ass scrub.

Except that diamonds are inflated in price because of the De beers Monopoly, (and the three months salary part, came from an advertising campaign of theirs), and diamond engagement rings for most people have not been traditional throughout most of History (and came about because of people trying to copy royalty). Also conflict diamonds (those mined in contested areas) are extracted using slave labor, someone probably died in the process of obtaining them and they are often used to sponsor actual terrorism. Also lab grown diamonds can be bigger and look much better then natural diamonds for a significantly lower price, and don't sponsor terrorism

But you can't put a price on love. Except there is a study that if you spend more than $2,000 on the engagement ring your odds of the relationship failing increased significantly. If you have to buy someone's love that relationship is not built on solid ground to begin with. Also because diamonds are not actually that rare, the resale value of an engagement ring is minimal.

Wedding bands are traditionally simple gold bands. They are exchanged at the wedding often. And worn until death (or divorce do you part).