Which among the following statements is not correct with respect to allotropes of Carbon ?

Statement D is incorrect because graphite is a very soft and slippery substance, not the second hardest. In graphite, carbon atoms are arranged in hexagonal layers held together by weak Van der Waals forces, allowing the layers to slide over each other. This property makes it an effective lubricant rather than a hard material.

• Option A: Graphite is a good conductor of electricity because each carbon atom is bonded to only three others, leaving one free valence electron to conduct charge.
• Option B: Diamond is the hardest known natural substance due to its rigid, three-dimensional tetrahedral covalent bonding.
• Option C: Fullerenes (such as C60 or Buckminsterfullerene) are a well-recognized class of carbon allotropes.

Since the question asks for the statement that is not correct, Option D is the correct choice.