Diamonds that are man-made[1] are stronger (fewer imperfections) than those that are mined from the earth. Because of this, industrial diamonds tend to be man-made. (for reference, industrial diamonds when cutting hard materials, such as metals).
While working on these diamonds, GE decided to start investigating making consumer level diamonds that could be sold for jewelry. They were able to produce diamonds that would have excelled when compared to natural diamonds (when it comes to the 4 Cs). One of the fun things was they could add various gases to the manufacturing process to create diamonds of various colors. There is still a decent cost associated with producing diamonds this way, so they probably would have still been expensive, but not at the levels that De Beers was charging at the time.
At this point GE started to look into what would happen if they would have actually gone down this line, selling consumer-level diamonds. After a little investigation, the GE lawyers and upper-management decided to kill off the idea as it would not have been worth the hassle. De Beers started a small campaign that was discrediting man-made diamonds, and it would have gotten a lot worse if GE even tried to enter the market. GE decided it was not worth the hassle, and killed the consumer-level diamond project.
De Beers has created an artificial market and they are doing what they can to prevent anyone else from entering their market. Most companies don't want to deal going up against them, so they just leave De Beers to run around gouging consumers.
[1] http://en.wikipedia.org/wiki/Synthetic_diamond#High_pressure...