Diamond is amazingly dense - much denser than graphite, which forms near the earth's surface. This implies formation at high pressure. Diamond is extraordinarily hard - while graphite, also made of carbon, is exceedingly soft. Hardness is the measure of a substance's resistance to being scratched. Diamond is the hardest substance known.
Durability is the capacity to withstand abuse.
The relative resistance to fracture is another. Although diamond is extraordinarily hard and is not fragile, it has a plane of weakness in its structure, a cleavage, along which it can be split. All substances, even diamond, can break or shatter. It is the result of its great dispersion - the refraction and separation of white light into rainbow colors. Diamonds are cold to the touch.