Diamonds are forever they say, and this is why: diamonds are the hardest substance known to exist. Nothing can tarnish a diamond. But how did they come to be so tough?

It Starts With An Atom

Everything is made from atoms, and these atoms are held together with chemical bonds. Something's hardness is determined by how tightly its atoms stick together.

For example, when a diamond scratches against a piece of metal, metal atoms are torn loose while the atoms in the diamond stay snugly in place. Why do the atoms of a diamond stick together so tightly?

Bonding

All atoms form only a limited number of chemical bonds at one time. For example, hydrogen atoms form only a single bond. Diamonds are made from carbon atoms which can form four bonds at once the maximum number for any atom.

This means that each atom in a diamond is chemically linked to four of its neighbors in a rigid pattern. This is like tying something down with four pieces of rope instead of just one or two it makes each carbon atom very secure.

There are other kinds of atoms that make four chemical bonds, but none of them form substances as hard as a diamond. This is because carbon is the smallest atom that can join to four of its neighbors. Its tiny size lets the carbon atoms snuggle more closely together than other atoms can, creating a firmer, harder material.

High Temperatures

It takes very high temperatures and pressures to pack carbon atoms tight enough to make a diamond. Without these, carbon atoms form materials like coal or graphite. Once they've been pressed into a diamond however, those carbon atoms are there to stay!