Diamond is the substance found in nature. Lets delve into why;

Structure of Diamond; Diamonds are made up entirely of carbon atoms organized in a covalent structure. Within this structure each carbon atom forms covalent bonds with four other carbon atoms resulting in a highly rigid and stable lattice.
Strength of Bonds; Covalent bonds rank among the resilient chemical bonds and the massive covalent structure in diamonds renders them exceptionally resistant to scratching or breaking.
Diamonds boast a 10 rating on the Mohs hardness scale, which is used in mineralogy to gauge mineral scratch resistance. This signifies that no other natural material can leave a scratch on a diamond.

Here are some intriguing details about diamonds and their hardness;

Diamonds find utility in settings such as cutting tools and drill bits owing to their extraordinary toughness.
The process of cutting and polishing diamonds necessitates the use of diamond dust due, to their toughness.
Although diamonds exhibit hardness they can sustain chips or fractures when subjected to sufficient force at a specific angle because of the orientation of their crystal lattice.
While there exist highly durable natural substances they might not enjoy as much recognition or accessibility.
If you want an example consider Lonsdaleite, a type of graphite that is expected to be tougher than diamond. Its quite uncommon and only discovered in meteorites. Ultimately when it comes to the natural substance, on Earth diamond takes the lead!