Diamond engagement rings date back to 1477, and diamond jewelry dates back much further. It was viewed as very exclusive/rare. Diamond engagement rings for the masses was from DeBeers advertising starting in 1938.

The growth in diamond mine production in the late 1800s tremendously increased diamond supply. DeBeers took advantage of historical perception of rarity and actual lower cost of materials to create a new market.

No, diamonds were actually kind of second-rate for a long time, medieval jewelry tended to favor flashier colored stones like rubies, emeralds, etc.

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

Although to be fair, this was in part because diamonds are hard to cut and they don't look good without faceted cuts. Diamonds only started becoming popular in the late 1800s when South African mines started producing them in bulk and modern cuts were developed, and they went mainstream once the De Beers marketing engine kicked in after WW2.

Likewise. My wife now wears my late mother's wedding ring, after we had it remade to fit her. (Resizing was non-trivial, as it is not a simple circle.) It's not hugely valuable, but it has a unique character and history.

(Added twist: the jeweller couldn't have it ready in time for the ceremony, so we made do with a dime-store trinket, and swapped it later.)

The most compliments I ever got on my spouses ring is in the couple months after our first child was born and her fingers were too swollen to wear the original wedding rings. She switched to a $10 Target ring with a huge fake diamond.

Likewise. There are some beautiful rings in antique stores that are unlike anything really being made today. Different craft.

It's also nice to think that the ring has had a life of its own already and you get to be another chapter in it.