Platinum Stone  

To make a long story short, they are antiques now.

A long time ago, when the game ended at tier 5, items such as these were VERY rare raid mob drops, that would be used by master tradeskillers with special recipes to make very special crafted items for players.

Since then, the recipes have been taken out of the game, even if the drops have not. There has been some speculation and rumormongering that the recipes may return some day, so folks that have these items tend to hold onto them. (I have a T5 version which I still have because it's essentially valueless) but they really dont serve much purpose.

Hope this helps.