Diamond is a form of carbon that crystallizes in the crystal system of highest symmetry known as the cubic system. It possesses a hardness far surpassing that of any other substance known in nature. The durability of a gem depends on both its hardness and toughness. Diamond, although highest on the scale of hardness, is not as tough as some gems because of its good cleavage. Cleavage is the tendency of a diamond to split in certain directions where the carbon atoms are furthest apart. Diamonds have a very high degree of transparency, refractivity and dispersion or 'fire' which gives rise in cut diamonds to a high degree of brilliancy and a display of prismatic colours. A diamond's fiery brilliance makes it cherished above all other gemstones by the majority of people. Diamonds occupy a position of incomparable demand.