Not only that, but you couldnt flood the market with diamonds even if you wanted to. Most of the true large diamonds are privately owned and stashed away. DeBeers, Zales, etc. only release a small amount each year to keep the prices high. Diamonds are overpriced, but badass.
Cubic Zirconias are fake man-made diamonds. They are perfect. I think it rarely occurs in nature, so its obvious to any good jeweler if a diamond is real or not.