It is HOWEVER one of the longest running and most successful marketing campaigns ever.
Diamonds are not an investment, they have no intrinsic value. Go ahead, take your diamond engagement ring to a jeweler the day after you buy it, you'll be lucky if they'll offer you half what you paid for it. The value goes down from there. Unless it's an antique who's status can be verified it's worthless.
2 months salary? Invented by DeBeers. Not a custom at all.
Surprise engagements? Also Invented by DeBeers. Jewelers couldn't help but notice that men shopping alone would spend more money than if a couple shopped together.
Think about it, most people were DIRT POOR for most of human history. You think sheep hearders bought diamond rings? You think families running their own farm in the middle of europe had money for diamonds? The upper class didn't do love marriages, Dad made an arrangement with some other "elite" family and there was no getting down on one knee and handing over a ring. It was more like "Guess what, you're engaged! To this random guy you haven't even met! Or this jerkwad that you don't even like!"