Convincing people they need a carbon rock through a brilliant marketing campaign ("A Diamond is Forever" named best advertising slogan of 20th century by Advertising Age). This slogan convinces Americans and global consumers to change their courtship practices; that it is completely normal and expected to spend 2 months salary on a diamond engagement ring as a symbol of love.
Another ad urged consumers to hold on to family diamond Jewelry as heirlooms. This cutoff the aftermarket of diamonds and increased the market power of the firm.
De Beers then artificially keeps the prices very high. These carbon rocks aren't rare in the natural world. The company restricts supply through a cartel system. Less than 200 companies are allowed to buy from the firm.