Diamond is the hardest known natural material, and its name actually comes from the ancient Greek for ‘unbreakable’. They are so hard because of the way that their atoms are bonded to each other. Have a look here:
Notice that the bonds between the carbon atoms in a diamond are firstly very strong (they are covalent bonds, meaning the atoms share electrons), and are arranged in a rigid 3D network – giving the diamond strength in all directions. There are no weak links, and nowhere for different layers to slide apart from each other (like in graphite, also made of carbon).