Diamond is one of the natural allotropes of carbon (the main allotrope being graphite; see also allotropes of carbon).

Properties
Diamond is a transparent, optically isotropic crystal with a refractive index of 2.417, a high dispersion of 0.044, and a specific gravity of 3.52.

Hardness and crystal structure
The diamond crystal bond structure gives the gem its hardness and differentiates it from graphite.Sometimes known as adamant, it is the hardest known naturally occurring material, scoring 10 on the old Mohs scale of mineral hardness. The material boron nitride, when in a form structurally identical to diamond, is nearly as hard as diamond; a currently hypothetical material, beta carbon nitride, may also be as hard or harder in one form. The diamond derives its name from the Greek adamas, "untameable" or "unconquerable", referring to its hardness.

Diamonds typically crystallize in the cubic crystal system and consist of tetrahedrally bonded carbon atoms. A second form called lonsdaleite with hexagonal symmetry is also found. The local environment of each atom is identical in the two structures. Cubic diamonds have a perfect octahedral cleavage, which means that they have four cleavage planes. Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles. Other forms include dodecahedra and cubes. Diamonds are commonly found coated in nyf, a gum-like skin. Their fracture may be step-like, conchoidal (shell-like, similar to glass) or irregular.

Optical properties
The lustre of a diamond is described as adamantine, which simply means diamond-like. Diamonds exhibit fluorescence of various colors under long wave ultra-violet light, but generally bluish-white, yellowish or greenish fluorescence under X-rays. Diamonds have an absorption spectrum consisting of a fine line in the violet at 415.5 nm. Colored stones show additional bands. Brown diamonds show a band in the green at 504 nm, sometimes accompanied by two additional weak bands also in the green. Adamas Gemological Laboratory (http://www.gis.net/~adamas) makes spectrophotometer machines that can distinguish natural, artificial, and color-enhanced diamonds.

Electrical properties
Except for most natural blue diamonds which are semiconductors, diamond is a good electrical insulator, but unlike most insulators, is a good conductor of heat because of the strong bonding within the molecule. Specially purified artificial diamonds have the highest thermal conductivity (20-25 W/cmK, five times more than copper) of any known solid at room temperature. Most natural blue diamonds contain boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductance. Natural blue diamonds recently recovered from the Argyle mine in Australia have been found to owe their color to an overabundance of hydrogen atoms: these diamonds are not semiconductors.

Thermal properties
Because diamonds have such high thermal conductance they are already used in semiconductor manufacture to prevent silicon and other semiconducting materials from overheating. Natural blue diamonds containing boron and synthetic diamonds doped with boron are p-type semiconductors. If an n-type semiconductor can be synthesized, electronic circuits could be manufactured of diamond. Worldwide research is in progress, with occasional successes reported, but nothing definite. In 2002 it was reported in the journal Nature that researchers have succeeded in depositing a thin diamond film on a diamond surface which is a major step towards manufacture of a diamond chip. In 2003 it was reported that NTT developed a diamond semiconductor device[1] (http://www.eetimes.com/at/hpm/news/OEG20030822S0005).

Composition and color
Type I diamonds have nitrogen atoms as the main impurity. If they are in clusters they do not affect the diamond's color (Type Ia). If dispersed throughout the crystal they give the stone a yellow tint (Type Ib), the Cape series. Typically a natural diamond crystal contains both Type Ia and Type Ib material. Synthetic diamonds which contain nitrogen are Type Ib.

Type II diamonds have no nitrogen impurities. Rarely, they contain no other impurities: these are Type IIa, colored pink, red or brown by structural anomalies arising through plastic deformation. Type IIb are the natural blue diamonds which contain scattered boron within the crystal matrix.

Diamonds occur in a variety of colors - steel, white, blue, yellow, orange, red, green, pink, brown and black. Colored diamonds contain impurities or molecular defects that cause the coloration, whilst pure diamonds are always transparent and colorless.

In the late 18th century, diamonds were demonstrated to be made of carbon by the rather expensive experiment of igniting a diamond (by means of a burning-glass) in an oxygen atmosphere and showing that carbonic acid gas (carbon dioxide) was the product of the combustion. The fact that diamonds are combustible bears further examination because it is related to an interesting fact about diamonds. Diamonds are carbon crystals that form deep within the Earth under high temperatures and extreme pressures. At surface air pressure (one atmosphere), diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (δH = -2 kJ / mole). So, despite De Beers' ad campaign, diamonds are definitely not forever. However, owing to a very large kinetic energy barrier, diamonds are metastable; they will not decay into graphite under normal conditions.

The diamond industry
Only diamonds are hard enough to cut other diamonds. Polishing and mounting add further value, brilliance, and appeal as jewelry.Due to their high dispersion and unsurpassed hardness, diamonds have long been prized as a constituent of jewellery. A large trade in gem-grade diamonds exists, mostly controlled by the De Beers company, which has used its monopoly to manipulate prices. At one time it was thought over 80% of the world's rough diamonds passed through the Diamond Trading Company (DTC, a subsidiary of De Beers) in London, but presently the figure is estimated at c. 60%. In the late 90's, the Canadian prospectors discovered several rich sources of diamonds. For example, the Ekati Diamond Mine, which was opened in 1998, produces 3 million carats of rough diamond every year. The Diavik Diamond Mine was opened in 2004.

Diamonds are valued according to the four C's of diamond grading, namely cut, clarity, color, and carat. Both rough and cut diamonds are graded and separated based on these four characteristics at a number of heavily guarded grading centers, such as the DTC.

Cut
The history of diamond cutting can be traced to the late Middle Ages, before which time diamonds were enjoyed in their natural octahedral state. The first "improvements" on nature's design involved a polishing of the crystal faces—this was called the point cut. Later still, a little less than one half of the crystal would be sawn off, creating the table cut. Neither of these early cuts would reveal what diamond is prized for today; its strong dispersion or fire. At the time, diamond was valued chiefly for its brilliant lustre and superlative hardness; a table-cut diamond would appear black to the eye, as they do in paintings of the era.

After 1676 the rose cut came into use by Belgian cutters: this was the first truly multi-faceted cut, with upwards of 16 facets. It is likely the rose cut is a design adopted from India, as many of the historical Indian diamonds were fashioned in this manner (although less symmetrically, as Indian cutters sought to minimize wastage).

Roughly 1900, the development of diamond saws and good jewellery lathes enabled the development of modern diamond cuts, chief among them the round brilliant cut. In 1919, Marcel Tolkowsky analyzed this cut. His calculations took both brilliance (the amount of white light reflected) and fire into consideration, creating a delicate balance between the two. His geometric calculations can be found in his book on Diamond Design (http://www.folds.net/diamond/index.html).

The modern round brilliant consists of 58 facets (or 57 if the culet is excluded); 33 on the crown (the top half above the middle or girdle of the stone) and 25 on the pavilion (the lower half below the girdle). In recent decades, most girdles are faceted. Many girdles have 32, 64, 80, or 96 facets; these facets are not counted in the total. While the facet count is standard, the actual proportions (crown height and angle, pavilion depth, etc.) are not universally agreed upon. One may speak of the American cut or the Scandinavian standard (Scan. D.N.), to give but two examples.

Even with modern techniques, the cutting and polishing of a diamond crystal always results in a dramatic loss of weight; rarely is it less than 50%. The round brilliant cut is preferred when the crystal is an octahedron, as often two stones may be cut from one such crystal. Oddly shaped crystals such as macles are more likely to be cut in a fancy cut—that is, a cut other than the round brilliant—which the particular crystal shape lends itself to.

Popular fancy cuts include the baguette (from the French, resembling a loaf of bread), marquise or navette ("little boat"), princess (square outline), heart, briolette (a form of the rose cut), and the pear or drop cuts. Generally speaking, these "fancy cuts" are not held to the same strict standards as Tolkowsky-derived round brilliants. Cuts are influenced heavily by fashion; baguettes—which accentuate a diamond's lustre and downplay its fire—were all the rage during the Art Deco period, whereas the princess cut—which accentuates a diamond's fire rather than its lustre—is currently gaining popularity. The princess cut is also popular amongst diamond cutters: of all the cuts, it wastes the least of the original crystal.

In the 1970s, Bruce Harding developed another mathematical model for gem design. Since then, several groups have used computer models (e.g., MSU, OctoNus (http://www.cutstudy.com), GIA (http://www.gia.org), and folds.net (http://www.folds.net/diamond/index.html)) and specialized scopes to design diamond cuts.

During the 1990s Israeli interests acquired about 20% of the diamond trade, buying diamonds from Russia and from mines in Africa not controlled by De Beers. De Beers now deals only in diamonds from their own mines. A major diamond cutting industry has grown up in Gujarat State, India where 90% of the world's diamonds (as measured by number of diamonds) are cut by a workforce of 800,000[2] (http://www.vibrantgujarat.com/sp-gems.html). Small diamonds previously not worth cutting are cut in India, opening up a new market segment for small diamonds.

Some cuts are:
Round
Radiant
Pear
Marquise
Emerald
Oval
Heart
Princess

The choice of cut is often decided by the original shape of the rough stone, location of the inclusions and flaws to be eliminated, the preservation of the weight, popularity of certain shapes amongst consumers and many other considerations. As far as the shape of the cut is concerned, it is very much a personal taste and preference. However, when jewelers judge the quality of a cut diamond, they often rate "Cut" as the most important of the "4-Cs." The key is not about the shape, but how good a job the cutters executed for a given shape. The proportion, symmetry and quality of the polish are essential criteria of a good cut. Since the "brilliance" and "fire" of a diamond depends very much on the angle of the facets in relation to each other. A poorly cut diamond with facets missed by a few degrees from ideal will result in a stone that lacks the gem quality. For a round brilliant cut, there is a balance between "brilliance" and "fire". When a diamond is cut for too much "fire", it would look like a cubic zirconia which gives out much more "fire" than real diamond. A well executed round brilliant cut should reflect most light out from the tabletop and make the diamond appear white when viewed from the top. An inferior cut will produce a stone that appears dark at the center and in some extreme cases the ring settings may show through the top of the diamond as shadows.

Sometimes the cutters compromise for less than ideal proportion and symmetry in order to avoid inclusions or to preserve the carat rating. Since the per-carat price of diamond is much higher when the stone is over one carat, many one-carat diamonds are the result of compromising "Cut" for "Carat". Some jewelry experts advise consumers to buy a .99 carat diamond for its better price or buy a 1.10 carat diamond for its better cut. A 1.00 carat diamond is usually poorly cut stone.

The "Cut" of the "4-Cs" is the most difficult part for a consumer to choose in selecting a good diamond because a GIA certificate will only show the measurements without a subjective ranking of how good the cut was. The other 3-Cs can be ranked simply by the rating in each category. It requires a trained eye to see the quality of a good "cut".

Clarity
Clarity is a measure of internal structural imperfections called "inclusions". Grades of clarity, which are mostly those used by Gemological Institute of America (GIA), are:

FL - "flawless" in that no inclusions are visible under 10 times magnification
IF - "internally flawless" with no inclusions visible under 10 times magnification, only small blemishes
VVS1 and VVS2 - "very very small" inclusions that are difficult to see under 10 times magnification. VVS1 is a better grade than VVS2.
VS1 and VS2 - "very small" inclusions and visible under magnification but invisible to the naked eye.
SI1 and SI2 - "small inclusions" that can be noticeable to the naked eye, if you know where to look.
"SI3" is a grade sometimes used in the industry, originally popularized by the European Gemological Laboratory (EGL) Los Angeles grading office. While theoretically a range including lower SI2 and upper I1, it's commonly used to mean I1's which are "eye clean", that is, which have inclusions which aren't readily visible to the naked eye. Neither the GIA nor the American Gemological Society (AGS), the most reputable well known US labs, assign this grade.
I1, I2 and I3 - "imperfect" and visible to the naked eye. For I3, the inclusions impact the brilliance of the diamond and are large and obvious.
All grades reflect the appearance to an experienced grader when viewed from above at 10x magnification, though higher magnifications and viewing from other angles are used during the grading process. In "colorless" diamonds, dark inclusions will tend to create the greatest drop of clarity grade. In other colors pale inclusions may have greater relief (may stand out more) and may cause a greater drop in grade.

Beyond the clarity grading terms, other considerations include the type, size and location of the "inclusion". Inclusions near or on the surface may weaken the diamond structurally. Depending on where the inclusion occurs in the cut diamond and how it is to be used, it may be possible to hide the inclusion behind the setting.

Laser "drilling" involves using a laser to burn a hole to a colored inclusion, followed by acid washing to remove the coloring agent. The clarity grade is the grade after the treatment. The treatment is considered permanent and both the GIA and AGS will issue grades for laser drilled diamonds. Reputable vendors should disclose that laser drilling has been used.

Clarity can also be "enhanced" by filling the fracture much like a car windshield crack can be treated. Such diamonds are sometimes called "fracture filled diamonds". Reputable vendors must disclose this filling and reputable filling companies use filling agents which show a flash of color, commonly orange or pink, when viewed closely. There is a significant price discount for fracture-filled diamonds. The GIA will not grade fracture-filled diamonds, in part because the treatment isn't as permanent as diamond. Reputable companies often provide for repeat treatments if heat causes damage to the filling. The heat required to cause damage is that of a blowtorch used to work on settings, and it is essential to inform anyone working on a setting if the diamond is fracture-filled, so they can apply cooling agents to the diamond and use greater care while working on it.

Color
Jewelers set diamonds in groups of similar colors.The Gemological Institute of America uses as "D" to "Z" scale for color where "D" is colorless and "Z" is yellow:

colorless: D, E, F
near colorless: G, H, I, J
faint yellow or brown: K, L, M
very light yellow or brown: N, O, P, Q, R
light yellow or brown: S, T, U, V, W, X, Y, Z

Colorless diamonds are priced higher than yellow diamonds. However, when a diamond's color is more intense than the "Z" grading, it enters the realm of "Fancy Color". In this case, the intensity of the color in the diamond plays a major role in its value. The value of a Fancy Color Diamond may far surpass that of colorless diamonds, if the intensity of the color is high and the color is rare. A diamond may come in all colors of the rainbow.

A fancy brown diamond may have low value, relative to colorless diamond. However, a fancy pink or blue diamond will command higher prices. Fancy-colored diamonds such as the deep blue Hope Diamond are particularly valuable.

Brown rather than yellow as the color became more common as Australian diamonds entered the market and is generally less appreciated by consumers and sold at a greater discount if the color is readily visible.

80% of the diamonds produced are poorer quality (discolored, less transparent) diamonds which are used as industrial diamonds, where their extreme hardness is useful in cutting and grinding otherwise intractable materials (including other diamonds). Lately, gas-phase deposition processes have been devised that allow thin diamond films to be grown on some surfaces, greatly increasing the durability of some machine tools.

While the prices are higher for colorless diamonds, the exact color most valued by a consumer is a matter of personal preference, with some preferring the very transparent D-F range, while others prefer the "warmer" colors in the G-J range and still others prefer a clearly visible tint.

Sources
Historically diamonds were found in alluvial deposits in southern India which are now worked out. Most diamond deposits are in Africa, notably in South Africa, Namibia, Botswana, the Republic of the Congo and Sierra Leone. Revolutionary groups in some of those countries have taken control of diamond mines, using the conflict diamonds to finance their operations.

There are also commercial deposits in the Northwest Territories of Canada, the Russian Arctic, Brazil and in Northern and Western Australia. Occasionally diamonds have been found in glacial deposits in Wisconsin and Indiana. The Wisconsin finds can be explained by recent Canadian discoveries, but the diamonds found in Indiana must have come from an as yet undiscovered source in Quebec as the movement of ice was from northeast to southwest. Tiny nanometer sized diamonds, often called nanodiamonds, are also found as presolar grains in primitive meteorites.

Diamonds have been manufactured artificially for over fifty years, and very recently companies began marketing them to the public as jewelry and in technology. For more information see Artificial diamond.

A city of major importance in diamond trade is Antwerp, Belgium. It is estimated that nearly 90% of the world's rough diamonds, 50% of cut diamonds, and 40% of industrial diamonds trade hands in Antwerp. The industry is represented by the Diamond High Council (HRD). Before Antwerp the port city of Bruges saw most diamond trade, holding its position since the 13th century. Toward the 15th century Bruges declined, its port choked with silt.

Antwerp had been the world centre of diamond trade since the 16th century, until the city's 1585 capture by the Spanish. Amsterdam then supplanted Antwerp as a trading centre, until the latter's resurgence beginning in the 19th century.

Symbolism of diamonds
It is said the Greeks believed diamonds were tears of the gods; the Romans believed they were splinters of fallen stars. Many long dead cultures have sought the divine or the mystical in diamond, thereby explaining its specialities.

Perhaps the earliest symbolic use of diamonds was as the eyes of Hindu devotional statues. The diamonds themselves were thought to be endowments from the gods and were therefore cherished. The point at which diamonds assumed their divine status is not known, but early texts indicate they were recognized in India since at least 400 BC.

In western culture, diamonds are the traditional emblem of fearlessness and virtue. Although rarely seen in jewellery prior to the Baroque period, early examples of betrothal jewels incorporating diamonds include the Bridal Crown of Blanche (ca. 1370-1380) and the Heftlein brooch of Vienna (ca. 1430-1440), a pictorial piece depicting a wedding couple.

Today, diamonds are used to symbolize eternity and love, being often seen adorning engagement rings. This modern tradition can be directly traced to the marketing campaigns of De Beers, starting in 1938. These campaigns have included measures such as:

showing diamonds as wedding gifts in popular romantic movies
publishing stories in magazines and newspapers which would emphasize the romantic value of diamonds and associate them with celebrities employing fashion designers and other trendsetters to promote the trend on radio and, later, television
enlisting the Royal Family of the United Kingdom to directly promote diamonds.

This campaign was described by De Beers' PR agency N. W. Ayer as "a new form of advertising which has been widely imitated ever since" with "no brand name to be impressed on the public mind. There was simply an idea -- the eternal emotional value surrounding the diamond." Indeed, the campaign succeeded in reviving the American diamond market, which had been weakened by "competitive luxuries", and in opening new markets where none had existed before. In Japan, for example, diamonds were successfully promoted as a western symbol of status, which coincided with Japan's cultural opening after World War II. Japan, which had no diamond tradition before the De Beers campaign, is today the second largest market for retail diamonds.

The slogan "A Diamond is Forever", invented by N.W. Ayer, is one of the most successful slogans in marketing history. Its purpose is to dissuade women from selling the diamonds they have received, so that prices can remain at a high level even at times of economic hardship.

The diamond engagement ring is, however, not an original invention of De Beers. It can be traced to the marriage of Maximilian I (then Archduke of Austria) to Mary of Burgundy in 1477. While the act did much to advance the Habsburg empire, it did little to make the diamond ring a widely encountered expression of betrothal.

The inception of the engagement ring itself can be tied to the Fourth Lateran Council presided over by Pope Innocent III in 1215. Innocent declared a longer waiting period between betrothal and marriage; plain rings of gold, silver or iron were used earliest. Gems were more than baubles; they were important and reassuring status symbols to the aristocracy. Laws were passed to preserve a visible division of social rank, ensuring only the privileged wore florid jewels. As time passed and laws relaxed, diamonds and other gems became obtainable to the middle class.

The LifeGem company further taps modern symbolism by offering to synthetically convert the carbonized remains of people or pets into "memorial diamonds." However, many people still feel very uncomfortable at the thought of wearing the carbonized remains of people as jewelry.

Related terms
A 'schlenter' is Australian or South African mining slang for 'fake', that is, an imitation diamond.

Famous diamond cutters
Gabriel Tolkowsky - A famous diamond cutter, who cut two of the world's largest diamonds, the Centenary Diamond and the Golden Jubilee Diamond. He has also attempted to copyright and/or patent various diamond designs.

Famous stones
The Allnatt Diamond
Centenary Diamond
Cullinan Diamond
The Deepdene
Dresden Green Diamond
Eugenie Blue Diamond
The Golden Jubilee
Great Chrysanthemum Diamond
The Heart of Eternity Diamond
Hope Diamond
Hortensia Diamond
Idol's Eye
Koh-i-Noor
Millennium Star
The Moussaieff Red Diamond
The Ocean Dream Diamond
The Orloff
Portuguese Diamond
Premier Rose Diamond
The Pumpkin Diamond
The Regent Diamond
Star of Africa
The Steinmetz Pink Diamond
The Taylor-Burton Diamond
The Tiffany Yellow Diamond
The Sancy
Vargas
and an unusual case:

BPM 37093, a degenerate star in the constellation Centaurus, which contains the largest known diamond in the universe: 1×1034 carats and 4,000 km in diameter