This is a "longstanding tradition" that was invented out of whole cloth in the 1930's and 1940's, by a series of De Beers advertising campaigns involving product placement with the Kardashian analogues of that era, culminating in the "A Diamond Is Forever" radio ads. Shaming America's male and female populace into buying into their cartel, the 'customary price' they recommended for diamond rings to demonstrate love rose continuously, not only in absolute terms, but as a share of personal income. It's an arms race with both zero objective value to anyone, and significant negative externalities.