The hardness of diamond comes due to the tetrahedral structure of diamond!!
Carbon element is a non metal and even though diamond in the hardest substance known,it is an allotrope of carbon and thus is A NON METAL!!
Diamond and graphite + fullerenes are allotropes of carbon,CARBON IS A NON METAL ALREADY PROVED BY THE SCIENTISTS LONG AGO!
elements which are brittle ,generally non-conductors,non-malleable and non-ductile are called non-metals. But there are exceptions like diamond.Thus it being a non-metal
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