A diamond necklace in a Harrods department store in London.Leon Neal | Afp | Getty Images"A diamond is forever," but perhaps not for the increasing number of consumers spurning the gemstone for lab-grown counterparts, gold and even other colored gemstones.The slogan was coined by diamond giant De Beers in 1948, capturing the impression of security and romance. But not all relationships withstand the test of time.The company's largest shareholder Anglo American plans to divest De Beers as it rest...