http://www.smithsonianmag.com/science-nature/diamonds-on-demand.html?c=y&page=2 said:
Natural diamonds aren't particularly rare. In 2006, more than 75,000 pounds were produced worldwide. A diamond is a precious commodity because everyone thinks it's a precious commodity, the geological equivalent of a bouquet of red roses, elegant and alluring, a symbol of romance, but ultimately pretty ordinary.
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Credit for the modern cult of the diamond goes primarily to South Africa-based De Beers, the world's largest diamond producer. Before the 1940s, diamond rings were rarely given as engagement gifts. But De Beers' marketing campaigns established the idea that the gems are the supreme token of love and affection. Their "A Diamond Is Forever" slogan, first deployed in 1948, is considered one of the most successful advertising campaigns of all time. Through a near total control of supply, De Beers held almost complete power over the diamond market for decades, carefully hoarding the gemstones to keep pricesand profitshigh.