What is the hardest naturally occurring substance on Earth?

Diamonds are the hardest naturally occurring substances on Earth due to their unique crystal structure. Each carbon atom in a diamond is tetrahedrally bonded to four other carbon atoms, forming a very strong and stable three-dimensional network. This arrangement makes diamonds incredibly resilient to scratching and damage, which is why they are often used in cutting and drilling tools, in addition to their popularity in jewelry.

Other substances like gold, granite, and quartz, while they have their own properties and uses, do not match the hardness of diamonds. Gold is a soft metal and can be easily scratched; granite is a dense and tough rock but not as hard as diamonds; quartz, while it is harder than many minerals, has a hardness of 7 on the Mohs scale, compared to diamond's score of 10. This distinction in hardness is crucial in geological applications and material science, emphasizing the exceptional nature of diamonds in terms of hardness.