The short answer is, diamonds are controlled by a cartel with literal warehouses of diamonds kept off the market to maintain artificial scarcity, compounded by very successful marketing to increase the perceived value above actual value, raising demand. If you are buying new diamonds, you are always paying too much, although how much extra varies.
Here’s an informative video on the subject: https://youtube.com/watch?v=PYUMKRwJzNg&feature=share9
The short answer is, diamonds are controlled by a cartel with literal warehouses of diamonds kept off the market to maintain artificial scarcity, compounded by very successful marketing to increase the perceived value above actual value, raising demand. If you are buying new diamonds, you are always paying too much, although how much extra varies.