[JoshuaEddy]

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.

[Clewza313]

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.