Diamond

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

Diamond crystal bond structure

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.

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.

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.

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

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%.

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.

Cutting

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.

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, 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 (as measured by number of diamonds) are cut by a workforce of 800,000[1]. Small diamonds previously not worth cutting are cut in India, opening up a new market segment for small diamonds.

Clarity

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 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 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".

Color

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

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.

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, 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 indistinguishable from the natural occurring ones, by 2005. The traditional diamond industry is evaluating countermeasures (source: [2]).

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 idols. 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 recognised 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 is widely attributed to the marketing campaigns of De Beers: Though the company may be responsible for popularizing the practice, especially in countries where such a tradition did not exist before—particularly in Japan—the diamond engagement ring 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 Hapsburg 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[3] company further taps modern symbolism by offering to synthetically convert the carbonized remains of people or pets into "memorial diamonds."

Related terms

Famous stones

• Cullinan Diamond
• Hope Diamond
• Koh-i-noor
• Millennium star

External links

• On conflict diamonds (UN)
• Article in Nature on the diamond chip
• Article in Nature on advancing techniques of growing diamond crystals
• OctoNus Software has posted several diamond cut studies, by various authors. OctoNus, Moscow State University, Bruce Harding, and others have posted work there.
• Gemological Institute of America
• Adamas Gemological Laboratory makes spectrophotometer machines that measure the color of gems. The machines can be programmed to distinguish natural, artificial, and color-enhanced gems.
• Wired article on diamond manufacture
• Diamond and Fullerenes models by Vincent Herr
• Learning about diamonds
• Chemical & Engineering News article on man made diamonds.

Further reading

• Diamond Design, Marcel Tolkowsky. Web edition as edited by 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)

Referenced By