A diamond is crystallized carbon, like the graphite in a lead pencil. Carbon is  the fundamental building block of all matter on earth, including humans. Diamonds are the hardest, most durable substance (10 on  Mohs' scale)  ever  discovered by man, and also the purest substance that occurs in nature. They are the product of tremendous heat and pressure, caused when the earth was still being formed.

The diamond is formed at a temperature and pressure so high it is equivalent to that existing 150 miles below the earth's surface. The diamond has a melting point of approximately 4,000 degrees centigrade, which is two and a half times greater than the melting point of steel. 

Don't confuse hardness with brittleness. While it is true that nothing except another diamond can cut or scratch a diamond, a sharp blow from a hard object can crack or shatter a diamond.

The worlds largest diamond ever found is the Cullinan (Star of Africa), it was found in 1905 at the Premier mine near Pretoria, South Africa. Its weight is (or was) 3,106 carats (about 1 1/3 lb). In 1908 it was cleaved into two parts and cut into 9 major stones and 96 small ones, for a total weight of 1,036 carat.

The famous Koh-i-noor diamond (meaning Mound of Light), is believed to be found in India some 5,000 years ago. It is the private property of the English Royal Family.