Diamond is pure carbon in a cubic crystal structure, in which every atom is covalently bonded to four neighbours in a rigid tetrahedral framework. That structure is the entire explanation of its properties: extreme hardness, very high thermal conductivity, high refractive index and strong dispersion. Graphite is the same element in sheets, and is one of the softest materials known. The difference between the hardest and one of the softest substances in nature is arrangement, not composition.
Diamond sits at 10 on the Mohs scale, which is a ranking of scratch resistance and not a linear measure, the gap between 9 and 10 is far larger than between any other adjacent pair. Hard is not the same as tough: diamond has perfect cleavage in four directions and can be split cleanly with a well-placed blow, which is how diamonds were shaped before modern sawing.
The Greek adamas, unconquerable or unbreakable, is the root of both diamond and adamant. The word was not originally specific to this stone and was applied to any exceptionally hard material, which is another instance of an ancient gem name that cannot be assumed to mean what it now means. Pliny's account of adamas includes claims, that it resists blows on an anvil, that it is softened by goat's blood, that do not describe diamond accurately, and the goat's blood story travelled through the lapidary tradition for well over a thousand years.
Until the eighteenth century essentially all diamonds came from India, principally the Golconda region; Brazilian discoveries followed, and the South African finds from the late 1860s changed the scale of the trade completely.