Diamond is the most precious substance in the world. It is also the hardest material and a pure carbon. It cannot be cut by metal. Diamond can be cut only by a diamond.

Diamond is a pure crystalline form of carbon which is obtained from mines. At 9000 Celsius it starts burning slowly and combines with oxygen to form carbon dioxide. At 10000 C it gets converted into graphite. At higher temperature the rate of formation of graphite is faster.

Diamond is a good conductor of heat but bad conductor of electricity. Its thermal conductivity is five times more than copper. Until 1955 diamond was obtained only from mines, but now diamond can be made by synthetic methods, this type of diamonds are called artificial diamond. Africa is the largest source of natural diamonds. About 95% of these come from this continent. Before they come to the market for sale, they are cut into different shapes and polished. The shine of diamond is not spoiled even after hundred of years.

Synthetic or Artificial diamonds were made for the first time in the world by the General Electric Co. of USA in 1995. In this process diamonds are produced from graphite. In high temperature furnaces graphite is heated to about 30000 C, under high pressure. By doing so graphite is converted into diamond. Synthetic diamond resembles the natural diamond in many respects. These are generally used in Jewelry and other industries.

The wholly colourless variety of diamond is the best quality diamond. Diamonds are chiefly given a slight colouring of black, blue, brown, green, pink, purple or red. Blood red is the rarest colour in diamonds.

Cullinan is the largest diam- ond in the world and it is named after the owner of the mine in South Africa. Its weight is 0.6 kg and it was discovered in 1905.