Diamond is cubic native carbon with the same composition as graphite, but with different structure. It is the hardest mineral (10 on the Mohs' scale), with the highest refractive index of 2.417 among all transparent minerals, and has a high dispersion of 0.044. Diamonds are brittle. Under UV light, the diamond frequently exhibits luminescence with different colors. It has a density of 3.52 g/cm³. The mass of diamonds is measured in carats; 1 carat=0.2 grams. Diamonds rarely exceed 15

The Hope Diamond is one of the largest diamonds in the world. © Richard T. Nowitz/Corbis. Reproduced by permission.

carats. Diamonds are insoluble in acids and alkalis, and may burn in oxygen at high temperatures.

Nitrogen is the main impurity found in diamonds, and influences its physical properties. Diamonds are divided into two types, with type I containing 0.001–0.23% nitrogen, and type II containing no nitrogen. If nitrogen exists as clusters in type I diamonds...