They don't just look purer, they are purer!

The problem is that rich people have been fleeced by diamond sellers for so long that they over value the "story" of a diamond (IE, carbon forged into a diamond over millennia etc etc. as a symbol for love blah blah).

The only reason they're expensive is artificial scarcity. De Beers only let a limited supply on the market each year. Diamonds themselves really aren't that rare at all