D colour flawless Winston Legacy stone sold at Christie's Geneva.
Company offers Argyle diamond jewels to celebs to wear at the Oscars.
Diamond miner affected by work stoppages and variable grades of stone.
Global contingent of almost 300 buyers invited to international fair.
Global diamond demand to grow 6% by 2020 with solitaires set to rise.
Changes reflect introduction of Supplier of Choice strategy.
American company unveils first design idea mimicking engagement ring.