Encyclopedia
- For other uses, including the shape ?, see Diamond .
Diamond is the hardest known natural material and one of the two best known forms of
carbon, whose
hardness and high dispersion of
light make it useful for industrial applications and
jewelry. Diamonds are specifically renowned as a
mineral with superlative physical qualities — they make excellent abrasives because they can be scratched only by other diamonds, Borazon,
ultrahard fullerite, or aggregated diamond nanorods, which also means they hold a polish extremely well and retain luster. About 130 million carats are mined annually, with a total value of nearly
USD $9 billion. About 100,000 kg are synthesized annually.
The name “diamond” derives from the ancient Greek
adamas . They have been treasured as
gemstones since their use as religious
icons in
India at least 2,500 years ago—and usage in
drill bits and
engraving tools also dates to early human history. Popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns. They are commonly judged by the “four Cs”:
carat,
clarity,
color, and
cut. Although
synthetic diamonds are produced each year at nearly four times the rate of natural diamonds, the vast majority of synthetic diamonds produced are small imperfect diamonds suitable only for industrial-grade use.
Roughly 49% of diamonds originate from central and southern
Africa, although significant sources of the mineral have been discovered in
Canada,
India,
Russia,
Brazil, and
Australia.
They are generally mined from
volcanic pipes, which are deep in the Earth where the high pressure and temperature enables the formation of the crystals. The mining and distribution of natural diamonds are subjects of frequent controversy—such as with concerns over the sale of
conflict diamonds by African paramilitary groups. There are also allegations that the
De Beers Group misuses its dominance in the industry to control supply and manipulate price via
monopolistic practices, although in recent years the company's market share has dropped to below 60%.
Material properties
A diamond is a transparent
crystal of
tetrahedrally bonded carbon atoms. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme
hardness of diamond, its high dispersion index, and high thermal conductivity.
Mechanical properties
Crystal structure
Diamonds typically crystallize in the face-centered
cubic crystal system and consist of
tetrahedrally bonded carbon atoms. The
unit cell of diamond has a two atom basis at and , which means half of the atoms are at lattice points and the other half are offset by , where 1 is the length of a side of the unit cell. Diamond's density is 3.52 g·cm
−3.
The tetrahedral arrangement of atoms is the source of many of diamond’s properties. The carbon atoms in
Graphite, the other major
allotrope of carbon, display a different connectivity and as a result shows dramatically different physical characteristics: graphite is a soft, dark gray, opaque mineral. Other elements of the carbon group such as
silicon crystalize like diamond.
Lonsdaleite is a polymorph of diamond that crystallizes with hexagonal symmetry. It is rarely found in nature but is characteristic of
synthetic diamonds. A cryptocrystalline variety of diamond is called carbonado. A colorless, grey or black diamond with a tiny radial structure is a spherulite.
Hardness
Diamond is the
hardest natural material known, scoring 10 on the relative Mohs scale of mineral hardness and having an absolute hardness value of between 90, 167, and 231 gigapascals in various tests. Diamond's hardness has been known since antiquity, and is the source of its name. However, aggregated diamond nanorods, an
allotrope of
carbon first synthesized in 2005, are now believed to be even harder than diamond.
The hardest diamonds in the world are from the
New England area in
New South Wales,
Australia. These diamonds are generally small, perfect to semiperfect octahedra, and are used to polish other diamonds. Their hardness is considered to be a product of the crystal growth form, which is single stage growth crystal. Most other diamonds show more evidence of multiple growth stages, which produce inclusions, flaws, and defect planes in the crystal lattice all of which affect their hardness .
Industrial use of diamonds has historically been associated with their hardness; this property makes diamond the ideal material for cutting and grinding tools. It is one of the most known and most useful of more than 3,000 known minerals. As the hardest known naturally occurring material, diamond can be used to polish, cut, or wear away any material, including other diamonds. Common industrial adaptations of this ability include diamond-tipped drill bits and saws, or use of diamond powder as an abrasive. Other specialized applications also exist or are being developed, including use as
semiconductors: some blue diamonds are natural semiconductors, in contrast to most other diamonds, which are excellent electrical
insulators. Industrial-grade diamonds are either unsuitable for use as gems or synthetically produced, which lowers their price and makes their use economically feasible. Industrial applications, especially as
drill bits and
engraving tools, also date to ancient times.
The hardness of diamonds also contributes to its suitability as a
gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well, keeping its luster over long periods of time. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching—perhaps contributing to its popularity as the preferred gem in an
engagement ring or
wedding ring, which are often worn every day.
Toughness
Unlike hardness, which only denotes resistance to scratching, diamond's toughness is good in cleavage directions, exceptional in all other directions. Toughness relates to a material's ability to resist breakage from forceful impact. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond is therefore more fragile in some orientations than others.
Color
Diamonds can occur in nearly any color, though yellow and brown are by far the most common. "Black" diamonds are not truly black, but rather contain numerous dark inclusions that give the gem its dark appearance. Diamonds with a detectable hue other than yellow or brown are known as
colored diamonds. If the color is strong enough, a stone may be referred to as a
fancy colored diamond by the trade. Colored diamonds contain impurities or structural defects that cause the coloration, while pure or nearly pure diamonds are transparent and colorless. Most diamond impurities replace a carbon atom in the
crystal lattice. The most common impurity,
nitrogen, causes a slight to intense yellow coloration depending upon the type and concentration of nitrogen present. The color scale for colorless diamonds goes from 'D' to 'Z' .
Thermodynamic stability
Like other forms of carbon like
coal, diamonds will burn at approximately 800 degrees
Celsius, providing that enough oxygen is available. The combustibility of diamond was shown in the late 18th century and previously described during
Roman times. The diamond phase of carbon is
metastable with respect to the graphitic phase under normal conditions; that is, graphite is thermodynamically favored over diamond . However, the rate of conversion from diamond to graphite is extremely slow due to the presence of a large kinetic barrier to this rearrangement. At room temperature, it would take an extremely long time for an appreciable amount of diamond to decay into graphite.
Electromagnetic properties
Optical properties
Diamonds exhibit a high dispersion of visible light. This strong ability to split white light into its component colors is an important aspect of diamond's attraction as a gemstone, giving it impressive prismatic action that results in so-called
fire in a well-cut stone. The luster of a diamond, its
adamantine brilliance, is a consequence of its high refractive index of 2.417 , which allows
total internal reflection to occur easily.
Some diamonds exhibit
fluorescence of various colors under long wave
ultraviolet radiation. Most diamonds show no fluorescence although colored diamonds show a wider range of fluorescence than the blue fluorescence normally observed in clear diamonds. Nearly all diamonds fluoresce bluish-white, yellow, or green under shorter
X-ray radiation. X-ray screening is used extensively in mining to separate the diamond-bearing from the non-fluorescing waste rock.
Thermal properties
Unlike most electrical insulators, diamond is a good conductor of heat because of the strong covalent bonding within the crystal. Most natural blue diamonds contain
boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductivity. Specially purified synthetic diamond has the highest thermal conductivity of any known solid at room temperature. Because diamond has such high thermal conductance it is already used in semiconductor manufacture to prevent silicon and other semiconducting materials from overheating. The
band gap of diamond is 5.4 - 6.4 eV.
Media
Natural history
Formation
Diamonds are formed by prolonged exposure of carbon bearing materials to high
pressure and temperature. On
Earth, the formation of diamonds is possible because there are regions deep within the Earth that are at a high enough pressure and temperature that the formation of diamonds is
thermodynamically favorable. Under
continental crust, diamonds form starting at depths of about 150 kilometers , where pressure is roughly 5 gigapascals and the temperature is around 1200 degrees Celsius . Diamond formation under
oceanic crust takes place at greater depths because of higher temperatures, which require higher pressure for diamond formation. Long periods of exposure to these high pressures and temperatures allow diamond crystals to grow larger.
Through studies of carbon isotope ratios , it has been shown that the carbon found in diamonds comes from both inorganic and organic sources. Some diamonds, known as
harzburgitic, are formed from inorganic carbon originally found deep in the Earth's
mantle. In contrast,
eclogitic diamonds contain organic carbon from organic detritus that has been pushed down from the surface of the Earth's crust through
subduction before transforming into diamond. These two different source carbons have measurably different
13C:
12C ratios. Diamonds that have come to the Earth's surface are generally very old, ranging from under 1 billion to 3.3 billion years old.
Diamonds occur most often as euhedral or rounded
octahedra and
twinned octahedra known as
macles or
maccles. As diamond's crystal structure has a cubic arrangement of the atoms, they have many
facets that belong to a
cube,
octahedron,
rhombicosidodecahedron,
tetrakis hexahedron or
disdyakis dodecahedron. The crystals can have rounded off and unexpressive edges and can be elongated. Sometimes they are found grown together or form double "twinned" crystals grown together at the surfaces of the octahedron. This is all due to the conditions in which they form. Diamonds are commonly found coated in
nyf, an opaque gum-like skin.
Diamonds can also form in other natural high-pressure, high-temperature events. Very small diamonds, known as
microdiamonds or
nanodiamonds, have been found in
impact craters where
meteors strike the Earth and create shock zones of high pressure and temperature where diamond formation can occur. Microdiamonds are now used as one indicator of ancient
meteorite impact sites.
Surfacing
Diamond-bearing rock is brought close to the surface through deep-origin
volcanic eruptions. The magma for such a volcano must originate at a depth where diamonds can be formed, 90 miles deep or more ; this is a relatively rare occurrence. These typically small surface volcanic craters extend downward into formations known as
volcanic pipes. The pipes contain material that was transported toward the surface by volcanic action, but was not ejected before the volcanic activity ceased. During eruption these pipes are open to the surface, resulting in open circulation; many
xenoliths of surface rock and even wood and/or
fossils are found in volcanic pipes. Diamond-bearing volcanic pipes are closely related to the oldest, coolest regions of continental crust . This is because cratons are very thick, and their
lithospheric mantle extends to great enough depth that diamonds are stable. Not all pipes contain diamonds, and even fewer contain enough diamonds to make mining economically viable.
The magma in volcanic pipes is usually one of two characteristic types, which cool into
igneous rock known as either
kimberlite or lamproite. The magma itself does not contain diamond; instead, it acts as an elevator that carries deep-formed rocks , minerals , and fluids upward. These rocks are characteristically rich in
magnesium-bearing
olivine,
pyroxene, and
amphibole minerals which are often altered to
serpentine by heat and fluids during and after eruption. Certain
indicator minerals typically occur within diamondiferous kimberlites and are used as mineralogic tracers by prospectors, who follow the indicator trail back to the volcanic pipe which may contain diamonds. These minerals are rich in
chromium or
titanium , elements which impart bright colors to the minerals. The most common indicator minerals are chromian
garnets , eclogitic garnets, orange Ti-pyrope, red high-Cr
spinels, dark
chromite, bright green Cr-
diopside, glassy green
olivine, black picroilmenite, and
magnetite. Kimberlite deposits are known as
blue ground for the deeper serpentinized part of the deposits, or as
yellow ground for the near surface smectite
clay and carbonate
weathered and
oxidized portion.
Once diamonds have been transported to the surface by magma in a volcanic pipe, they may
erode out and be distributed over a large area. A volcanic pipe containing diamonds is known as a
primary source of diamonds.
Secondary sources of diamonds include all areas where a significant number of diamonds, eroded out of their kimberlite or lamproite matrix, accumulate because of water or wind action. These include alluvial deposits and deposits along existing and ancient shorelines, where loose diamonds tend to accumulate because of their approximate size and density. Diamonds have also rarely been found in deposits left behind by glaciers ; however, in contrast to alluvial deposits, glacial deposits are not known to be of significant concentration and are therefore not viable commercial sources of diamond.
Diamonds can also be brought to the surface through certain processes which may occur when two
continental plates collide and deeply formed rock is thrust to the surface, although this phenomenon is less understood and currently assumed to be uncommon.
Gemological characteristics
The use of diamonds as gemstones of decorative value is the most familiar use to most people today, and is also the earliest use, with decorative use of diamonds stretching back into antiquity. The dispersion of white light into a rainbow of colors, known in the trade as
fire, is the other primary characteristic of gem diamonds, and has been highly prized throughout history. Over time, especially since around 1900, experts in the field of
gemology have developed methods of characterizing diamonds and other gemstones based on the characteristics most important to their value as a gem. Four characteristics, known informally as the
four Cs, are now commonly used as the basic descriptors of diamonds: these are
carat,
clarity,
color, and
cut.
Most gem diamonds are traded on the wholesale market based on single values for each of the four Cs; for example knowing that a diamond is rated as 1.5 carats, VS2 clarity, F color, excellent cut, is enough to reasonably establish an expected price range. More detailed information from within each characteristic can then be used to determine actual market value for individual stones. Consumers who purchase individual diamonds are often advised to use the four Cs to pick the diamond that is "right" for them; to these is sometimes added the "fifth C" of
cost.
Other characteristics not described by the four Cs can and do influence the value or appearance of a gem diamond. These characteristics include physical characteristics such as the presence of
fluorescence, as well as data on a diamond's history including its source and which gemological institute performed evaluation services on the diamond.
Cleanliness also dramatically affects a diamond's beauty.
There are four major gemological associations which "certify" diamonds: that is, define the four Cs of a diamond. While carat weight and cut angles are mathematically defined, the clarity and color are judged by the trained human eye and are therefore open to slight variance in interpretation.
- Gemological Institute of America was the first laboratory to issue modern diamond reports, and holds the highest reputation amongst gemologists for its consistent, conservative grading.
- American Gemological Society is not as widely recognized nor as old as the GIA, but garners an equally high reputation.
- International Gemological Laboratory is a generally respected laboratory but suffers from a negative industry reputation for its grading practices, which are perceived by critics as being either less conservative or less consistent than the GIA and AGS.
- European Gemological Laboratory USA Good reputation among the trade for consistent and fair grading. Not as widely known as GIA or AGS.
Carat
The
carat weight measures the mass of a diamond. One carat is defined as a fifth of a gram, or exactly 200
milligrams . The
point unit—equal to one one-hundredth of a carat —is commonly used for diamonds of less than one carat. All else being equal, the value of a diamond increases exponentially in relation to carat weight, since larger diamonds are both rarer and more desirable for use as gemstones. A review of comparable diamonds available for purchase in September 2005 demonstrates this effect :
| Carat size | Cost per carat | Total cost |
|---|
| 0.5 carat | 3,000 | 1,500 |
| 1.0 carat | 6,500 | 6,500 |
| 1.5 carats | 8,500 | 12,750 |
| 2.0 carats | 13,000 | 26,000 |
| 3.0 carats | 17,000 | 51,000 |
| 5.0 carats | 23,000 | 115,000 |
The price per carat does not increase smoothly with increasing size. Instead, there are sharp jumps around milestone carat weights, as demand is much higher for diamonds weighing just more than a milestone than for those weighing just less. As an example, a 0.95 carat diamond may have a significantly lower price per carat than a comparable 1.05 carat diamond, because of differences in demand.
A weekly diamond price list, the Rapaport Diamond Report , is published by Martin Rapaport, CEO of Rapaport Group of New York, for different diamond cuts, clarity and weights. It is currently considered the de-facto retail price baseline. Jewelers often trade diamonds at negotiated discounts off the Rapaport price .
In the wholesale trade of gem diamonds, carat is often used in denominating lots of diamonds for sale. For example, a buyer may place an order for 100 carats of 0.5 carat, D–F, VS2-SI1, excellent cut diamonds, indicating he wishes to purchase 200 diamonds of those approximate characteristics. Because of this, diamond prices are often quoted per carat, rather than per stone.
Total carat weight is a phrase used to describe the total mass of diamonds or other gemstone in a piece of jewelry, when more than one gemstone is used. Diamond solitaire earrings, for example, are usually quoted in t.c.w. when placed for sale, indicating the mass of the diamonds in both earrings and not each individual diamond. T.c.w. is also widely used for diamond necklaces, bracelets and other similar jewelry pieces.
Clarity
Clarity is a measure of internal defects of a diamond called
inclusions. Inclusions may be crystals of a foreign material or another diamond crystal, or structural imperfections such as tiny cracks that can appear whitish or cloudy. The number, size, color, relative location, orientation, and visibility of inclusions can all affect the relative clarity of a diamond. The Gemological Institute of America and others have developed systems to grade clarity, which are generally based on those inclusions which are visible to a trained professional when a diamond is viewed from above, under 10x magnification.
Diamonds become increasingly rare when considering higher clarity gradings. Only about 20 percent of all diamonds mined have a clarity rating high enough for the diamond to be considered appropriate for use as a gemstone; the other 80 percent are relegated to industrial use. Of that top 20 percent, a significant portion contains a visible inclusion or inclusions. Those that do not have a visible inclusion are known as "eye-clean" and are preferred by most buyers, although visible inclusions can sometimes be hidden under the setting in a piece of jewelry.
Most inclusions present in gem-quality diamonds do not affect the diamonds' performance or structural integrity. However, large clouds can affect a diamond's ability to transmit and scatter light. Large cracks close to or breaking the surface may reduce a diamond's resistance to fracture.
Diamonds are graded by the major societies on a scale ranging from flawless to imperfect.
Color
A chemically pure and structurally perfect diamond is perfectly transparent with no
hue, or
color. However, in reality almost no gem-sized natural diamonds are absolutely perfect. The color of a diamond may be affected by chemical impurities and/or structural defects in the
crystal lattice. Depending on the hue and intensity of a diamond's coloration, a diamond's color can either detract from or enhance its value. For example, most white diamonds are discounted in price as more yellow hue is detectable, while intense pink or blue diamonds can be dramatically more valuable.
Most diamonds used as gemstones are basically transparent with little tint, or
white diamonds. The most common impurity,
nitrogen, replaces a small proportion of carbon atoms in a diamond's structure and causes a yellowish to brownish tint. This effect is present in almost all white diamonds; in only the rarest diamonds is the coloration due to this effect undetectable. The GIA has developed a rating system for color in white diamonds, from "D" to "Z" , which has been widely adopted in the industry and is universally recognized, superseding several older systems once used in different countries. The system uses a benchmark set of either natural diamonds of known color grade, or precision-crafted
cubic zirconia; test lighting conditions are also standardized and carefully controlled. Diamonds with higher color grades are rarer, in higher demand, and therefore more expensive, than lower color grades. Oddly enough, diamonds graded Z are also rare, and the bright yellow color is also highly valued. Diamonds graded D-F are considered "colorless", G-J are considered "near-colorless", K-M are "slightly colored". N-Y usually appear light yellow or brown.
In contrast to yellow or brown hues, diamonds of other colors are much rarer and more valuable. While even a pale pink or blue hue may increase the value of a diamond, more intense coloration is usually considered more desirable and commands the highest prices. A variety of impurities and structural imperfections cause different colors in diamonds, including yellow, pink, blue, red, green, brown, and other hues. Diamonds with unusual or intense coloration are sometimes labeled "fancy" by the diamond industry. Intense yellow coloration is considered one of the fancy colors, and is separate from the color grades of white diamonds. Gemologists have developed rating systems for fancy colored diamonds, but they are not in common use because of the relative rarity of colored diamonds.
Cut
Diamond cutting is the art and science of creating a gem-quality diamond out of mined rough. The
cut of a diamond describes the manner in which a diamond has been shaped and polished from its beginning form as a rough stone to its final gem proportions. The cut of a diamond describes the quality of workmanship and the angles to which a diamond is cut. Often diamond cut is confused with "shape."
There are mathematical guidelines for the angles and length ratios at which the diamond is supposed to cut at in order to reflect the maximum amount of light. Round brilliant diamonds, the most common, are guided by these specific guidelines, though fancy cut stones are not able to be as accurately guided by mathematical specifics.
The techniques for cutting diamonds have been developed over hundreds of years, with perhaps the greatest achievements made in 1919 by
mathematician and gem enthusiast Marcel Tolkowsky. He developed the round brilliant cut by calculating the ideal shape to return and scatter light when a diamond is viewed from above. The modern round brilliant has 57 facets , counting 33 on the
crown , and 24 on the
pavilion . The girdle is the thin middle part. The function of the crown is to diffuse light into various colors and the pavilion's function to reflect light back through the top of the diamond.
Tolkowsky defines the ideal dimensions to have:
- Table percentage = 53%
- Depth percentage = 59.3%
- Pavilion Angle = 40.75°
- Crown Angle = 34.5°
- Pavilion Depth = 43.1%
- Crown Depth = 16.2%
The culet is the tiny point or facet at the bottom of the diamond. This should be a negligible diameter, otherwise light leaks out of the bottom. Tolkowsky's ideal dimensions did not include a girdle. However, a thin girdle is required in reality in order to prevent the diamond from easily chipping in the setting. A normal girdle should be about 1%–2% of the overall diameter.
The further the diamond's characteristics are from Tolkowsky's ideal, the less light will be reflected. However, there is a small range in which the diamond can be considered "ideal." Today, because of the relative importance of carat weight in society, many diamonds are often intentionally cut poorly to increase carat weight. There is a financial premium for a diamond that weighs the magical 1.0 carat, so often the girdle is made thicker or the depth is increased. Neither of these tactics make the diamond appear any bigger, and they greatly reduce the sparkle of the diamond. So a poorly cut 1.0 carat diamond may have the same diameter and appear as large as a 0.85 carat diamond. The depth percentage is the overall quickest indication of the quality of the cut of a round brilliant. "Ideal" round brilliant diamonds should not have a depth percentage greater than 62.5%. Another quick indication is the overall diameter. Typically a round brilliant 1.0 carat diamond should have a diameter of about 6.5 mm. Mathematically, the diameter in millimeters of a round brilliant should approximately equal 6.5 times the
cube root of carat weight, or 11.1 times the cube root of gram weight.
Shape
Diamonds do not show all of their beauty as rough stones; instead, they must be cut and polished to exhibit the characteristic fire and brilliance that diamond gemstones are known for. Diamonds are cut into a variety of shapes that are generally designed to accentuate these features.
Diamonds which are not cut to the specifications of Tolkowsky's round brilliant shape are known as "fancy cuts." Popular fancy cuts include the
baguette ,
marquise,
princess ,
heart,
briolette , and
pear cuts. Generally speaking, these "fancy cuts" are not held to the same strict standards as Tolkowsky-derived round brilliants and there are less specific mathematical guidelines of angles which determine a well-cut stone. Cuts are influenced heavily by fashion: the baguette cut—which accentuates a diamond's luster and downplays its fire—was all the rage during the
Art Deco period, whereas the princess cut—which accentuates a diamond's fire rather than its luster—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. The past decades have seen the development of new diamond cuts, often based on a modification of an existing
cut. Some of these include extra facets. These newly developed cuts are viewed by many as more of an attempt at brand differentiation by diamond sellers, than actual improvements to the state of the art.
Quality
The quality of a diamond's cut is widely considered the most important of the four Cs in determining the beauty of a diamond; indeed, it is commonly acknowledged that a well-cut diamond can appear to be of greater carat weight, and have clarity and color appear to be of better grade than they actually are. The skill with which a diamond is cut determines its ability to reflect and refract light.
In addition to carrying the most importance to a diamond's quality as a gemstone, the cut is also the most difficult to quantitatively judge. A number of factors, including proportion,
symmetry, and the relative angles of various facets, are determined by the quality of the cut and can affect the performance of a diamond. A poorly cut diamond with facets cut only a few degrees out of alignment can result in a poorly performing stone. For a round brilliant cut, there is a balance between "brilliance" and "fire." When a diamond is cut for too much "fire," it looks like a
cubic zirconia, which gives off much more "fire" than real diamond. A well-executed round brilliant cut should reflect light upwards 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.
Several different theories on the "ideal" proportions of a diamond have been and continue to be advocated by professional gemologists. Recently, there has been a shift away from grading cut by the use of various angles and proportions toward measuring the performance of a cut stone. A number of specially modified viewers and machines have been developed toward this end. They included the FireScope, a.k.a. SymmetriScope or IdealScope , Hearts and Arrows Viewer , , , and ASET . These viewers and machines often help consumers determine the light performance results of the diamond in addition to the traditional 4 C's. Along with this shift there are a few companies that provide results on these viewers and machines in addition to the original 4c's. The GIA has also developed criteria for grading the cut of round brilliant stones.
The cutting process
The process of shaping a rough diamond into a polished gemstone is both an art and a science. 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. 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.
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%. Sometimes the cutters compromise and accept lesser proportions and symmetry in order to avoid inclusions or to preserve the carat rating. Since the per carat price of diamond shifts around key milestones , many one-carat diamonds are the result of compromising "Cut" for "Carat." Some jewelry experts advise consumers to buy a 0.99 carat diamond for its better price or buy a 1.10 carat diamond for its better cut, avoiding a 1.00 carat diamond which is more likely to be a poorly cut stone.
Light Performance
The light performance concept was first introduced by GemEx in 1998, then AGS introduced their version in 2005 and GIA introduced their version in 2006. The movement to understand and measure the light dynamics and behavior of a diamond is historic and probably the most significant event in modern diamond history that will affect how a diamond should be cut to maximize beauty. Prior to this time there was no technologies or protocol that could relate a diamond’s actual light performance with the proportions and angles that it was cut. Much of the notion on how much brilliance a round diamond could produce was based on theory not by measurement. For shapes other than round there was practically no reliable information available for the consumer.
Since everything about a diamond’s beauty has to do with how it shines, sparkles, and the light it emits then logically having accurate quantitative and qualitative analysis of the light being returned by a diamond can be very helpful in determining its beauty and evaluating its cut quality.
Light Performance
Light performance deals with how much light, what kind of light and the origin of light being emitted or reflected by a diamond when subject to a light source.
Performance has often been associated with cars, stereos, boats, watches, and computers, but rarely been it ever been used to describe a diamond. The degree a diamond is able to radiate light determines its beauty, desirability, and value. Thus it is important that its light performance be quantified and compared with other diamonds. Where proportions are used to forecast results light performance goes after the bottom line, the actual light output from a diamond. For the end users the information on the light performance of a diamond has the greatest relevance because they will know how much light they are getting for their money. This is analogous to knowing what the wattage of a light bulb is before one makes a purchase. Knowing how a product performs relative to others in the market is critical for consumers whether they are purchasing a cell phone, CD player or a truck.
Cleaning
Although it is not one of the four Cs,
cleanliness affects a diamond's beauty as much as any of the four Cs. A clean diamond is more brilliant and fiery than the same diamond when it is "dirty." Dirt or grease on the top of a diamond reduces its luster. Water, dirt, or grease on the bottom of a diamond interferes with the diamond's brilliance and fire. Even a thin film absorbs some light that could have been reflected to the person looking at the diamond. Colored dye or smudges can affect the perceived color of a diamond. Historically, some jewelers' stones were misgraded because of smudges on the girdle, or dye on the culet. Current practice is to thoroughly clean a diamond before grading its color.
Maintaining a clean diamond can sometimes be difficult, as jewelry settings can obstruct cleaning efforts, and oils, grease, and other hydrophobic materials adhere well to a diamond's surface. Some jewelers provide their customers with
ammonia-based cleaning kits;
ultrasonic cleaners are also popular.
Cleanliness does not affect the diamond's market value, as any competent jeweler will clean the diamond before offering it for sale. However, cleanliness might reflect a diamond's sentimental value: some jewelers have noted a correlation between ring cleanliness and marriage quality .
History
Diamonds are thought to have been first recognized and mined in
India, where significant alluvial deposits of the stone could then be found. The earliest written reference can be found in the
Buddhist text, the Anguttara Nikaya another
sanskrit text, the Arthashastra, which was completed around 296 BCE and describes diamond's hardness, luster, and dispersion. Diamonds quickly became associated with divinity, being used to decorate religious
icons, and were believed to bring good fortune to those who carried them. Ownership was restricted among various castes by color, with only kings being allowed to own all colors of diamond.
In February 2005, a joint
Chinese-U.S. team of
archaeologists reported the discovery of four
corundum-rich stone ceremonial burial
axes originating from China's Liangzhu and Sanxingcun cultures which, because of the axes' specular surfaces, the scientists believe were polished using diamond powder . Although there are diamond deposits now known to exist close to the burial sites, no direct evidence of coeval diamond mining has been found: the researchers came to this conclusion by polishing corundum using various lapidary abrasives and modern techniques then comparing the results using an
atomic force microscope. At that scale, the surface of the modern diamond-polished corundum closely resembled that of the axes; however, the polishes of the latter were superior.
Diamonds were traded to both the east and west of India and were recognized by various cultures for their gemological or industrial uses. In his work
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, the
Roman writer
Pliny the Elder noted diamond's ornamental uses, as well as its usefulness to
engravers because of its hardness. In China, diamonds seem to have been used primarily for engraving
jade and drilling holes in beads. Archaeological evidence from
Yemen suggests that diamonds were used as drill tips as early as the 4th century BCE. In
Europe, however, diamonds disappeared for almost 1,000 years following the rise of
Christianity because of two effects: early Christians rejected diamonds because of their earlier use in
amulets, and Arabic traders restricted the flow of trade between Europe and India.
Until the late
Middle Ages, diamonds were most prized in their natural octahedral state, perhaps with the crystal surfaces polished to increase luster and remove foreign material. Around 1300, the flow of diamonds into Europe increased via
Venice's trade network, with most flowing through the
low country ports of
Bruges,
Antwerp, and
Amsterdam. During this time, the taboo against cutting diamonds into gem shapes, which was established over 1,000 years earlier in the traditions of India, ended allowing the development of diamond cutting technology to begin in earnest. By 1375, a guild of diamond polishers had been established at
Nuremberg. Over the following centuries, various diamond cuts were introduced which increasingly demonstrated the fire and brilliance that makes diamonds treasured today: the
table cut, the
briolette , the
rose cut , and by the mid-17th century, the
Mazarin, the first brilliant cut diamond design. In 1919, Marcel Tolkowsky developed an
ideal round brilliant cut design that has set the standard for comparison of modern gems; however, diamond cuts have continued to be refined.
The rise in popularity of diamonds as gems seems to have paralleled increasing availability through European history. In the 13th century, King
Louis IX of France established a law that only the king could own diamonds. However, within a century diamonds were popular gems among the moneyed aristocratic and merchant classes, and by at latest 1477 had begun to be used in
wedding ring 
