Because diamond is the hardest substance found in this earth. It is a non metal and it has highest melting point.

It is hardest than the tungsten.

Each carbon atom is covalently bonded to four other carbon atoms Due to which A lot of energy is needed to separate the atoms in diamond. Because covalent bonds are strong, and diamond contains very many covalent bonds. This makes diamond's melting point and boiling point very high.

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