Why is diamond much harder than graphite even though both are made entirely of carbon?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — carbon group element Modern version of the periodic table of the elements (printable).

Diamond and graphite are both allotropes of carbon, but they possess vastly different physical properties due to their distinct crystal structures.1

In diamond, every carbon atom is covalently bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral lattice. This structure creates an infinite, highly stable network that can only be destroyed by breaking many strong bonds, resulting in extreme hardness.1

Conversely, graphite consists of parallel sheets of carbon atoms arranged in fused hexagonal rings. While the bonds within these layers are strong, the layers themselves are held together only by weak van der Waals forces. Because these layers can easily slide past one another, graphite is soft and slippery, contrasting sharply with the structural integrity of diamond.1