Question

how diamond is a non metal​

Answer 1

Diamond is an alotroph of Carbon
As Diamond is formed by Carbon atoms(sp³)
And Carbon is a metal, but when it forms SP³Carbon it loses some of its properties and becomes a Non Metal

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Answer 2

Diamond is an alotroph of carbon.which is a non metals . If we burn diamond under strong heating around3000 degree nothing is left and carbon dioxide evolves which gives evidence that it is made up of carbon.