Abstract: De Beers is a leading company of the century diamond industry, which is the core company of the "Diamond Empire" of Oppenheimer family. In Nov. 2011, the Oppenheimer family made a breathtaking decision which is to sell the stock right that they have of De Beers to its mother company Anglo American (AA.) and signed out the diamond industry. This paper researched that the inner connection between the change of diamond market and the decision of Oppenheimer family, and analyzed the reason of change by analyzing and comparing the market condition and the data of the international and domestic market around 2011. The present results showed that the market change for 2011-2015 is more likely a result of the combination of global financial crisis and the intensified inner industrial conflict, and whose time and the influence to the diamond market the family may be predicted. Obviously, that the family singed out is the result of the change of diamond market, instead of the reason. On May 29^th in 2018, De Beers established a new brand named Lightbox to produce and sell synthetic diamonds, which showed huge change of its market stratagy and the beginning of the new game of diamond market.