Diamond is Carbon
It may seem surprising that diamond is simply carbon, just like charcoal or graphite. In fact carbon has at least two other rare, and only recently discovered forms, or allotropes, known as fullerenes.
The difference is caused by the different types of bonding between adjacent atoms to form different types of crystalline structure.
In diamond, each carbon atom is bonded to four other carbon atoms in a tetrahedral structure, like a pyramid. Each link or bond is the same length, and the tetrahedral formation is therefore completely regular. It is the strength and regularity of this bonding which makes diamond very hard, non-volatile and resistant to chemical attack.
Theoretically a perfect diamond crystal could be composed of one giant molecule of carbon.
Carbon is a non-metallic element with the atomic number of 6, and an atomic Weight of 12. In combination with oxygen and hydrogen it is contained by all living objects. In the form of graphite it appears black or dark gray, opaque, and is very soft, whereas in the form of diamond is it clear, colourless, and extremely hard. In fact diamond is the hardest known naturally occurring substance.
Carbon has a density of about 2.3 and diamond about 3.5.

What Makes Diamond An Ideal Gemstone?
Diamond possesses many qualities which make it an ideal gemstone.
It is extremely hard, and also very tough and hard-wearing, and this also helps it to take a very high polish. Some hard articles are brittle which detracts from their durability. Only cubic boron nitride (borazon) is harder.
In its pure form it is colourless, has a high refractive index, so has a very high lustre.
It possesses high dispersion, meaning that different light wavelengths are diffracted differently, giving a strong scintillating play of prismatic colours.

Diamond Discoveries
Diamonds seem to have been known for about 3,000 years, being mentioned in Exodus chapter 28, however in early times, other hard minerals were often confused with diamond.
It is thought that the earliest diamonds were found in about the 12th century B.C., in India , which remained the most important, if not the sole, source until 1725, when diamonds were discovered in Brazil.
The Indian and Brazilian deposits had been almost exhausted when in 1866, the Eureka diamond was discovered in South Africa, followed by the Star of South Africa in 1869. Shortly afterwards, the great South African diamond rush had started, and South Africa remains one of the world's most important sources of diamonds today.
Diamonds have since been discovered in many other regions of the world, including Russia and Australia.
Until the South Africa finds, diamonds were so rare and valuable, that they were only owned by the very wealthy. They were not available in high street shops!