30s of research gives me this
http://www.google.co.uk/patents?id=0eBEAAAAEBAJ&zoom=4&dq=%2...It's a patent application from 1928 which speaks about engagement rings with a bezel for a stone.
I'm going out on a limb and saying that diamonds would be one choice for the stone prior to 1928. They certainly are only one amongst many choices in the present day too. Do USA-ians only have diamond engagement rings?
The cited Priceonomics article contains a logical flaw. It says demand was created in the De Beers campaign but then goes on to describe how great demand was purposefully under-supplied by Rhodes in order to prop up the price. Similarly it says roughly 'you like diamonds because a marketer told you to' but also then says that diamonds prior to establishing large mines were the preserve of Kings and Queens ... why then did they like them; they are just people too. The answer of course is that they're durable and look pretty and aren't [or weren't] commonly available. It's a great article, but they like you I fear have just pushed it that tiny little bit further seems supported.
Some other citations from a couple of minutes searching:
- this patent suggests that large settings on narrow bands were the fashion in 1927, http://www.google.co.uk/patents?id=LtZEAAAAEBAJ&zoom=4&dq=%2...
- Beck vs Cohen, 16 March 1933 concerns the specifics of engagement rings being part of an obligation to marry; suggesting they were a customary part of betrothals at that time.
- from 1868, http://books.google.co.uk/books?id=tL1YAAAAcAAJ&dq=%22engage..., suggests at pp18-19 that when betrothal fell out of fashion in the middle ages engagement rings were retained.
TL;DR - the suggestion in your post is that engagement rings were very uncommon [my word, AlexandrB's was "occassional"] in USA prior to a De Beers campaign. This seems to be an exaggeration. Perhaps De Beers instead achieved the end of making diamonds the fashionable stone to choose for the already customary engagement ring.