"The difference is that the world diamond market is largely controlled by a single private enterprise, the South Africa-based De Beers cartel. The geniuses behind De Beers recognized early on that a stable, profitable diamond industry depended on controlling both supply and demand. De Beers rarely discovers new sources of diamonds; rather, it focuses on controlling existing ones, limiting production, and if necessary buying up surplus gems and stockpiling them to prop up the market. It sets prices arbitrarily and cuts off supplies to dealers who buy through unauthorized channels. On the marketing side, De Beers hired advertising firms, starting with N.W. Ayer in the late 1930s, to render axiomatic the idea that diamonds = true love."

http://www.straightdope.com/columns/read/2524/is-a-diamonds-...

http://www.barnesandnoble.com/w/the-rise-and-fall-of-diamond...