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My wife has a diamond ring in her possession that my Great-Grandfather (a jeweler by trade) made for his wife my Great-Grandmother. It certainly predates the great depression, so it’s at least 96 years old. The universal use of a diamond ring is, as you say, a result of DeBeers advertising, but engagement rings were in widespread use long before that. It’s just that those rings might have different gems, not just diamonds.Nevertheless, I think it’s a good idea to deprecate expensive items like diamond engagement rings or fur streimels or silk bekeshes in order to reduce the expense and burden of weddings.
