Diamonds are neither rare, nor valuable. In one of the most successful ad campaigns of all time, De Beers claimed men should spend one month’s salary on a diamond ring. Then they raised their prices, restricted their supply, and created an illusion that generations of people still believe to be a mandatory part of the marriage process. Source
As a geologist and someone who can buy whole carats worth of diamonds (unpolished, obviously) for a fraction of the cost just by going to minerology sites instead of jewelry stores, I can confirm this.
And you can too! Just buy diamonds directly from the mine. They’re not as cheap as dirt but they’re not thousands of dollars.
before de beers, wedding bands were just solid gold or maybe had some sort of gem but that wasn’t too common.