Diamonds have specific and certain properties. Diamonds are a crystallized form of carbon, one of the most common elements in the world.
However they differ from other types of carbon due to the immense pressure under which they have been formed. It is this pressure that squeezes and concentrate the carbon and changes its form to that which we know of as diamond.
As mentioned before, diamonds are the crystallized form of carbon created under extreme heat and pressure. It's this same process that makes diamonds the hardest mineral we know of. A diamond ranks a 10 on the Mohs Hardness Scale (see below). It can be anywhere from 10 to hundreds of times harder than a mineral ranked nine on the Mohs scale, such as corundum. Corundum is a class of minerals that includes rubies and sapphires.
It is the molecular structure of diamonds that makes them so hard. Diamonds are made of carbon atoms linked together in a lattice structure. Each carbon atom shares electrons with four other carbon atoms, forming a tetrahedral unit. This tetrahedral bonding of five carbons forms an incredibly strong molecule. Graphite, another form of carbon, isn't as strong as diamond because the carbon atoms in graphite link together in rings, where each atom is only linked to one other atom.