What is considered the hardest natural substance found on Earth?

The hardest natural substance found on Earth is diamond. This is due to its unique crystalline structure, where carbon atoms are arranged in a strong tetrahedral lattice configuration. This structure allows diamonds to withstand significant pressure and resist scratching from other materials, making them exceptionally hard.

In contrast, while ruby and sapphire are both forms of corundum and are known for their hardness, they do not match the hardness of diamond. Coal, being a carbonaceous material, does not have the same structural integrity as diamonds and is significantly softer. Hence, diamonds are consistently recognized for their unparalleled hardness, making them the correct answer to the question.