The idea that De Beers kind of invented the engagement ring or even popularized it is not factual, though I see it crop up nowadays.

Emily Post, in her 1922 Etiquette discusses what's appropriate in wedding and engagement rings. She is, of course, writing for the better classes. For the poor, getting a wedding ring was challenge enough (lyrics to "Th One that I Love Loves Me"). It was an old ttradition that in that lesser situation the woman would wear the wedding band on her right hand, and move it to her left during the wedding.

I just finished reading a book from 1898 How to Cook Husbands that included a scene where the gentleman of interest produces a small box with a ring to indicate his matrimonial intentions. No details were required, this was so normal.

The use of a ring to signify marriage or betrothal remains culturally specific and differed widely, but there has been records of wedding rings, if not particular details, for a lot longer than the 1930s. First thing that comes to my head in the English-speaking tradition is probably Chaucer. At the end of the Clerk's tale Griselda returns her ring to her husband to end the marriage.

To yow broghte I noght elles, out of drede, But feith, and nakednesse, and maydenhede;

And heere agayn your clothyng I restoore,

And eek your weddyng ryng, for everemore.

"The remenant of youre jueles redy be

Inwith youre chambre, dar I saufly sayn.

Naked out of my fadres hous," quod she,

"I cam, and naked moot I turne agayn.

Another example of the diamond engagement ring being talked about earlier than the DeBeers advertising in the 1930s: in Anne of Green Gables, published in 1908, Anne talks about diamonds and how highly they are regarded, and what she thinks of them (the reality doesn't live up to her imagination).