The hardest known natural substance is diamond. It's a crystalline form of pure carbon with its atoms arranged in a tightly bonded cubic lattice structure.

This arrangement makes it extremely strong and resistant to scratching and abrasion. It's also very dense and has a high melting point, making it very durable.

Here's why diamond is considered the hardest:

* Mohs Hardness Scale: Diamonds have a Mohs hardness of 10, the highest on the scale. This scale measures a material's resistance to scratching.

* Vickers Hardness: Diamonds have a Vickers hardness of 9000 to 10,000 kg/mm², which measures resistance to indentation.

While other substances like lonsdaleite (hexagonal diamond) are theoretically harder than diamond, they are extremely rare and haven't been found in large enough quantities for practical use.