Diamond

Diamond is one of the natural allotropes of carbon (the main one being graphite).

Table of contents
1 Properties
2 The diamond industry
3 Symbolism of diamonds
4 Famous stones
5 External links
6 Further reading

Properties

Sometimes known as adamant, it is the hardest known naturally occurring material, scoring 10 on the old Mohs hardness scale. 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.

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.

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.

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.

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.

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.

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 though out 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 = -2KJmol-1). So, despite De Beers' ad campaign, diamonds are definitely not forever. However, owing to a very large kinetic energy barrier, diamonds will not decay into graphite under normal conditions.

The diamond industry

Due to their high dispersion, diamonds have been prized as a constituent of jewellery, and a large trade in gemstone-class diamonds exists, mostly controlled by the De Beers company, which has used its monopoly to control prices.

Marcel Tolkowsky's 1919 book on Diamond Design describes the history of diamond cutting since the late Middle Ages. Roughly 1900, the development of diamond saws and good jewelry lathes enabled the modern Round Brilliant cut. Tolkowsky determined a detailed design for this cut. His geometric calculations are in his book.

In the 1970s, Bruce Harding developed another mathematical model for gem design. Since then, several groups have used computer models (e.g., MSU, OctoNus, GIA, and folds.net) 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 are cut by a workforce of 800,000. Diamonds are valued according to the four C's of diamond grading, namely color, clarity, cut, and carat.

Clarity is a measure of internal structural imperfections called "inclusions". Grades of clarity used by Gemological Institute of America are:

  • FL - "flawless" in that no inclusions are visible under 10 times magnification
  • IF - "internally flawless" with no inclusios 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 are noticeable to the naked eye, if you know where to look. "SI3" is an unofficial grade sometimes used in the industry.
  • 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.

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. Clarity can be "enhanced" by filling the fracture much like a car windshield crack can be treated. Such diamonds are sometimes called "fracture filled diamonds".

The Gemological Institute of America uses as "D" to "Z" scale for color where "D" is colorless and "Z" is yellow:

  • colourless: D, E, F
  • near colorless: G,H, I, J
  • faint yellow: K, L, M
  • very light yellow: N, O, P, Q, R
  • light yellow: S, T, U, V, W, X, Y, Z
Colorless diamonds are priced higher than yellow diamonds. Diamonds of other colors may be priced higher than yellow diamonds and are graded "Z+". Fancy-colored diamonds such as the deep blue Hope Diamond are particularly valuable.

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.

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, Canada in 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 were first produced artificially on February 16, 1953 in Stockholm, Sweden by the QUINTUS project of ASEA, Sweden's major electrical manufacturing company using a bulky apparatus designed by Baltzar von Platen. Pressure was maintained within the device at an estimated 83,000 atmospheres for an hour. A few small crystals were produced. The discovery was kept secret.

While large diamonds have up to now been more expensive to produce artificially than to mine, smaller artificial diamonds and especially diamond dust have become an important industry with General Electric at the forefront. As of 2003, two companies planned to introduce high-quality artificial diamonds, visually indistinguishible from the natural occurring ones, by 2005. The traditional diamond industry is evaluating countermeasures (source: [1]).

A city of major importance in diamond trade is Antwerp, Belgium.

Symbolism of diamonds

Diamonds are the traditional emblem of fearlessness and are used to symbolize eternity and love, being often seen adorning engagement rings.

The "tradition" of diamonds as engagement rings was created by De Beers as a promotional tool, which they continue to use to this day. Contrary to popular belief, they were not used as engagement rings previous to De Beers's advertising campaign.

The LifeGem[1] company further taps this symbolism by offering to synthetically convert the carbonized remains of people or pets into "memorial diamonds."

Famous stones

External links

Further reading

  • Diamond Design, Marcel Tolkowsky. Web edition as edited by Jasper Paulsen. www.folds.net, Seattle, 2001.
  • The New Alchemists: Breaking Through the Barriers of High Pressure, Robert M. Hazen, Times Books, Random House, New York, 1992, hardcover, 286 pages, ISBN 0-8129-2275-1

See also: List of minerals, Diamonds (card suit)

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