Diamond is a mineral with a hardness of 10 out of 10 on the Mohs scale of mineral hardness [?]. These Cubicly structured gems are made of carbon, their full chemical compound being nothing more than C..
In fact Diamond is the only gem material comprised of a single chemical element: pure carbon, like graphite. But the atoms in diamond have been forced into a compact, three-dimensional structure by the high pressures in the upper mantle, where it is formed. Thus diamond is much harder and has a higher specific gravity (3.52) than graphite . There is nothing comparable to it in hardness; it is therefore nearly imperishable. It is resistant to any form of abrasion by other minerals. Its cutting resistance is 140 times greater than that or corundum.
However, the hardness of a diamond is different in the individual crystal faces. This allows one to cut diamond with diamond (or diamond powder, in which various diamond hardnesses occur).
It is usually colorless, but may be colored by yellow, red, brown, grey or black impurities. It has a brilliant adamantine to greasy luster. It is usually transparent, but can be translucent to opaque when very dark-colored. Its very strong luster enables the experienced eye to differentiate between a diamond and its imitations. X-rays are not transmitted through diamonds (in contrast to diamond-like minerals and synthetics).
A diamond is generally not sensitive to chemical attack. Only chromic sulphuric acid can transform it at 392 degrees Fahrenheit to carbon dioxide.
It is only meta-stable at room temperature and standard atmospheric pressure. In an inert atmosphere, diamond can safely be heated to at least 1500 C without damage. However, once the temperature exceeds 1800 C there is rapid transformation to graphite.
It occurs as isolated crystals, which may be in the form of a more or less perfect octahedron, or other more complex octahedron forms. The crystal can also be in the form of a rhombic dodecahedron, or a tetrahexahedron with rounded corners and slightly curved faces, to the point of being almost spherical. Basically triangular twinned forms are also frequent, but cubic forms are rare.Rough-looking surfaces display display superficial irregularities either in the form of fairly large cavities or hundreds of smaller irregularities, only recognizable under a lens. Pieces of diamond are often found that are clearly cleavages of other larger stones.
When the faces are curved or fairly rough, the crystals are generally translucent, even though it may be evident from cleavage surfaces that these imperfections are in an outer "skin" and that the crystals are transparent on the inside.
Transparent stones are usually colorless, but can be various shades of yellow to dull yellow, or more rarely, yellow with a brownish tinge. As an extreme rarity, there are diamonds that are blue, pale green, pink, violet, and even reddish. The strongest colors are usually confined to the less transparent, outer layer.
Diamond is mined from kimberlite and alluvial deposits, though gem diamonds did not crystallize directly from its host rock, but carried through them by volcanism. Kimberlites are rare and ultrabasic rocks which form rough cylindrical pipes or sometimes dykes, found in continental shield areas, and diamonds are also found in deposits in rivers running from such rocks.
The largest recorded diamond is the Cullinan, from the Transvaal in South Africa. Uncut it weighed 3106 carats.
The most important diamond fields are those of Kimberley (South Africa), also Tanzania, Yakutia, India, Brazil, and Namibia. Zaire is responsible for up to 60 percent of annual diamond production, mainly for industrial uses.
Diamonds are only said to be worth exploiting where they occur in average concentrations of one part in twenty million, or where twenty tons of rock have to be worked for each gram of diamond.
Its strength and hardness compared to other minerals, even of similar composition, is reflected in its name, which comes from the Greek word "Adamas," for "invincible," or "unconquerable." The optical properties of diamonds are exceptional: they make the stone the king of gems. It has been used for adornment since very early times.
There is still considerable uncertainty to the origin of diamond. Like ruby, diamond is a gemstone whose true geological source has been obscured by volcanic activity. The most widely accepted theory is that it was formed at greater depths in the earth's crust, or even mantle, at very high pressures and temperatures. Explosive types of volcanic phenomena would then have been responsible for driving it to the surface, with such a rapid drop in temperature that it was impossible for the diamond to be transformed into graphite, which is the carbon phase, stable at low pressures. It would presumably have been carried to the surface in kimberlite, which constitutes the infill of diamond-bearing pipes (structures with the appearance of explosive volcanic vents).
Microanalysis indicates that the oldest diamonds found so far are about 3300 Ma old, and stones of this age occur in the Kimberley and Finsch kimberlite pipes in South Africa.
Diamond was extremely rare up to the eighteenth century and was only fully appreciated after the modern type of brilliant cut, which shows it in all its glory, developed at the beginning of the 20th century.
For many centuries, the only place where diamonds were found was India, where very small quantities were mined. Early in the 18th century, diamonds also began to be mined in Brazil, which shortly afterwards became the principal world supplier. In the second half of the nineteenth century, diamond began to be mined from deposits in South Africa.
Since the beginning of the twentieth century, diamonds have been found in Angola, Zaire, Ghana, Guinea, Ivory Coast, Tanzania (one of the largest primary deposits in the world), and the Soviet republics. Diamonds are also found in Guyana, Venezuela, and in limited quantities, Borneo. They have been mined in China, and considerable quantities have been discovered in Australia.
During the last thirty years it has been recognized that there are various types of diamond which differ from each other in their absorption spectra, fluorescence, electric conductivity, and cleavage. Today, science differentiates between type Ia, Ib, IIa, and IIb.
Only 20 percent of all diamonds are suitable for use in jewellery. Gem-quality diamonds are colorless, transparent, and unflawed. Care has to be taken during mounting because of perfect cleavage. High temperatures induce etchings on the facets and care must be taken during soldering.
The market value of diamonds is determined by complex grading systems that divide them into a wide range of categories. Basic characteristics considered are weight, color, and purity. Less crucial factors are the proportions of the cut and surface finish.
Varieties of diamond are "bort," which has a rounded, fibrous, radiate structure, and "carbonado," which is black and microcrystalline. Both are used for industrial purposes only.
The majority of diamonds are used for drilling, cutting, and grinding-tools.
Low-quality diamonds are used as industrial abrasives.
At present the production of synthetic diamonds, which are not suitable for jewellery, is greater than the output of natural diamonds. High pressure high temperature (HPHT) methods now allow synthetic diamonds weighing a few carats to be grown on a commercial scale.
Today HPHT methods also enable the production of large synthetic diamonds for gem-quality or ornamental use. The color of crystals is normally yellow to brown. Colorless diamonds are difficult to produce, so current production of gem-quality synthetic diamonds is fancy yellow. They can also be given a pink color, using radiation and annealing.
Synthetic moissanite is manufactured to imitate the ornamental and gem properties of diamond, but the properties of moissanite may be detected easily by a gemologist with simple equipment. Zirconia is another imitation or synthetic diamond.