Here's why:

* Crystalline structure: Diamonds are made of carbon atoms arranged in a highly ordered, repeating three-dimensional pattern called a crystal lattice.

* Hardness: This regular arrangement of atoms gives diamonds their exceptional hardness, making them the hardest natural substance known.

* Other properties: Diamonds also exhibit high refractive index (which contributes to their sparkle), high thermal conductivity, and a very high melting point.