I think the diamod is purely to help potential new owners of pre-loved (or hated) platinum watches know it is platinum! I remembered i heard somewhere that many watches. vintage (1920s~1990s) are being passed off as platinum even though it is white gold. This is especially confusing as many of them have the exact same dial/hands/bezel/case designetc etc...
But in the next 15-30 years, i'm sure vintage (which means 'present' pateks) colletors would be happy there is a diamond to help them differentiate it! No more being dud-ed into buying a white gold!
Seriously, if the dial colour and everything else is the same, i.e the only difference is the CASE METAL, i'm sure 95% of us can't tell the metal is diamond or gold unless we have 2 different cased watches of the same make side by side!
but if you hold the wg in your hand, without having felt the platinum, how would u know which is which? 19.3 v 21.4g/cm3 sems too close a call without having both metals in front to diffrentiate.
i am sure if given a piece of metal with no indiction wg or pt, we would not tell the difference unless we have pior experience.
