Answers

2016-02-08T20:52:55+05:30
Magnesium is the hardest metal .
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2016-02-08T20:53:36+05:30
Diamond is the hardest metal...
And the metal which can break diamond into pieces is itself diamond
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actually , diamond is not a metal,for your kind information, but yes it can be said to be the hardest material(diamond is an allotroph of carbon) 
it is chromium