What is the hardest natural substance on Earth?

Diamond is recognized as the hardest natural substance on Earth due to its unique crystal structure and the strength of the covalent bonds between carbon atoms within its lattice. The hardness of a material is typically measured using the Mohs scale, where diamond has a rating of 10, the highest possible score. This exceptional hardness is a result of the way carbon atoms bond in a tetrahedral arrangement, creating a very strong and stable structure that resists scratching and abrasion.

In contrast, gold, quartz, and ruby have lower hardness ratings. Gold, while valuable and ductile, is much softer and can be easily scratched. Quartz, with a hardness of 7 on the Mohs scale, is hard but significantly less so than diamond. Ruby, which is a variety of corundum, has a hardness of 9 but still does not match the unparalleled hardness of diamond. Thus, diamonds stand out as the most robust natural material known, making them the correct answer to the question.