Platinum is eternally durable making it the perfect setting for diamonds.  Its natural whiteness makes diamonds dazzle securely.  White gold, of course, is not naturally white.  It is, in reality, yellow gold alloyed with other metals to achieve a white look.  This plating can wear away, thus revealing the yellow-grey colour of the white gold.  The more immediate means of distinguishing the two metals is their comparative weight.  Platinum is a dense metal and weighs 40% more than 18k white gold.  Even a simple wedding band made of platinum is noticeably heavier than the same ring made of white gold, giving reassurance of its value.