Diamonds were very valuable prior to the cartel coming on the scene, and diamonds or "carbonados" were mined and traded going back hundreds of years. DeBeers didn't transform them from trash to gems, they merely elevated the price a great deal. My crude estimates based on some research I did a while back was that the cartel has increased the price by about a factor of 3. However, that doesn't account for the increases in mining efficiency through the 20th century, which would have lowered the price of diamonds significantly, leading to a higher ratio of the current price versus a hypothetical fair market price in the present day.