Diamonds have been a good store of value, but have a history littered with interesting and repugnant milestones (cartels, blood diamonds), and may lose that perception as laboratory stones become more readily available. It is quite possible that the value of a given diamond will deteriorate below the cost of location, extraction, cutting and presentation over the next decade. Will gold be a store of value in a hundred years? Will bitcoin have any relevance in ten years?
Whilst governments as a matter of deliberate policy destroy the value of fiat currency, then individuals will always be looking for alternative means to store value in an attempt to be one step ahead of the thieves.