Fake or real diamond?

Diamond is a mineral composed of pure carbon. It is the hardest naturally occurring substance known and is also the most popular gemstone. Because of their extreme hardness, diamonds have a number of important industrial applications.

The hardness, brilliance, and sparkle of diamonds make them unsurpassed as gems. In the symbolism of gemstones, the diamond represents steadfast love and is the birthstone for April. Diamond stones are weighed in carats (1 carat = 200 milligrams) and in points (1 point = 0.01 carat). In addition to gem-quality stones, several varieties of industrial diamonds occur, and synthetic diamonds have been produced on a commercial scale since 1960.

Diamonds are found in three types of deposits: alluvial gravels, glacial tills, and kimberlite pipes. Only in kimberlite pipes, such as those at Kimberley, South Africa, are they present in the original rock in which they were formed, probably lying at depths of more than about 75 miles.

Diamonds found in alluvial and glacial gravels must have been released by fluvial or glacial erosion of the kimberlite matrix and then redeposited in rivers or in glacial till.

Diamonds vary from colourless to black, and they may be transparent, translucent, or opaque. Most diamonds used as gems are transparent and colourless or nearly so. Colourless or pale blue stones are most valued, but these are rare; most gem diamonds are tinged with yellow. A "fancy" diamond has a distinct body colour; red, blue, and green are rarest, and orange, violet, yellow, and yellowish green more common. Most industrial diamonds are gray or brown and are translucent or opaque, but better-quality industrial stones grade imperceptibly into poor quality gems. The colour of diamonds may be changed by exposure to intense radiation (as released in a nuclear reactor or by a particle accelerator) or by heat treatment.

A very high refractive power gives the diamond its extraordinary brilliance. A properly cut diamond will return a greater amount of light to the eye of the observer than will a gem of lesser refractive power and will thus appear more brilliant. The high dispersion gives diamonds their fire, which is caused by the separation of white light into the colours of the spectrum as it passes through the stone.

The scratch hardness of diamond is assigned the value of 10 on the Mohs scale of hardness; corundum, the mineral next to diamond in hardness, is rated as 9. Actually, diamond is very much harder than corundum; if the Mohs scale were linear, diamond's value would be about 42. The hardness of a diamond varies significantly in different directions, causing cutting and polishing of some faces to be easier than others.

In the atomic structure of diamond, as determined by X-ray diffraction techniques, each carbon atom is linked to four equidistant neighbours throughout the crystal. This close-knit, dense, strongly bonded crystal structure yields diamond properties that differ greatly from those of graphite, native carbon's other form.

Man-made (synthetic) diamonds are usually produced by subjecting graphite to very high temperatures and pressures. Synthetic diamond resembles natural diamond in most fundamental properties, retaining the extreme hardness, broad transparency (when pure), high thermal conductivity, and high electrical resistivity for which diamond is highly prized. Because synthesis is an expensive process, large stones of gem quality are rarely made. Instead, most synthetic diamond is produced as grit or small crystals that are used to provide hard coatings for industrial equipment such as grinding wheels, machine tools, wire-drawing dies, quarrying saws, and mining drills. In addition, diamond films can be grown on various materials by subjecting carbon-containing gas to extreme heat; these layers can be used in cutting tools, windows for optical devices, or substrates for semiconductors.

In 1880 the Scottish chemist James Ballantyne Hannay claimed that he had made diamonds by heating a mixture of paraffin, bone oil, and lithium to red heat in sealed wrought-iron tubes. In 1893 the French chemist Henri Moissan announced he had been successful in making diamonds by placing a crucible containing pure carbon and iron in an electric furnace and subjecting the very hot (about 4,000° C [7,000° F]) mixture to great pressure by sudden cooling in a water bath. Neither of these experiments has been repeated successfully.

During the first half of the 20th century the American physicist Percy Williams Bridgman conducted extensive studies of materials subjected to high pressures. His work led to the synthesis by the General Electric Company, Schenectady, N.Y., of diamonds in its laboratory in 1955. The stones were made by subjecting graphite to pressures approaching 7 gigapascals (1 million pounds per square inch) and to temperatures above 1,700° C (3,100° F) in the presence of a metal catalyst. Tons of diamonds of industrial quality have been made in variations of this process every year since 1960.

In 1961 shock-wave methods, or explosive-shock techniques, were first used to produce diamond powder, and small quantities of the material are still formed this way. Beginning in the 1950s, Russian researchers began to investigate methods for synthesizing diamond by decomposition of carbon-containing gases such as methane at high heat and low pressure. In the 1980s commercially viable versions of this chemical vapour deposition method were developed in Japan.

Cubic zirconia (or CZ), the cubic crystalline form of zirconium dioxide (ZrO2), is a mineral that is widely synthesized for use as a diamond simulant. The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors.

Because of its low cost, durability, and close visual likeness to diamond, synthetic cubic zirconia has remained the most gemologically and economically important diamond simulant since 1976. Like diamond, cubic zirconia is very hard (it can scratch glass) and has a high refractive index. In its clear form it closely resembles diamond. It can however be produced in a variety of colours and cuts.

From a distance it's very difficult to tell cubic zirconia from diamond. Even up close, it can be baffling for anyone other than an expert to distinguish the two without the use of specialist equipment.

When used as a substitute for diamond, CZ usually has a little less brilliance but slightly more prismatic fire. Modern-day production of CZ results in a stone that is almost flawless; something you won't see readily in nature. CZ is also a lot heavier than diamond.

It can also be noted that diamond is a thermal conductor while CZ is an insulator. This property can also be used to distinguish between the two using the right tools.