= Solution
In <crystal polymorphism>, the same <chemical composition> has different <crystal structures>; for an <chemical element>, this is a form of <allotropy>. <Diamond> has a three-dimensional tetrahedral network of <covalent bonds>, giving high <hardness> and a wide <band gap>. <Graphite> has strongly bonded planar sheets, with weaker interactions between sheets. Sliding between sheets makes it a useful <solid> <lubricant>; mobile electronic states give good electrical conduction within the sheets. Their different properties come from different bonding and structures, despite both being <carbon>.
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