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Courtesy of Wikpedia
Material properties of diamond and
A diamond is a transparent crystal of tetrahedrally bonded carbon atoms (sp3)
that crystallizes into the diamond lattice which is a variation of the face centered
cubic structure. Diamonds have been adapted for many uses because of the material's
exceptional physical characteristics. Most notable are its extreme hardness and thermal
conductivity (900—2,320 W·m-1·K-1), as well as wide bandgap and high
optical dispersion. Above 1,700 °C (1,973 K / 3,583 °F) in vacuum or oxygen-free
atmosphere, diamond converts to graphite; in air, transformation starts at ~700 °C.Naturally
occurring diamonds have a density ranging from 3.15—3.53 g/cm3, with very pure diamond
typically extremely close to 3.52 g/cm3. Despite the hardness of diamonds, the chemical
bonds that hold the carbon atoms in diamonds together are actually weaker than those
that hold together the other form of pure carbon, graphite. The difference is that
in diamonds, the bonds form an inflexible three-dimensional lattice. In graphite,
the atoms are tightly bonded into sheets, which can slide easily however.
Hardness
Diamond is the hardest natural material known, where hardness is defined as resistance
to scratching and is graded between 1 (softest) and 10 (hardest) using the Mohs scale
of mineral hardness. Diamond has a hardness of 10 (hardest) on this scale. Diamond's
hardness has been known since antiquity, and is the source of its name.
The diamond hardness depends on its purity, crystalline perfection and orientation:
hardness is higher for flawless, pure crystals oriented to the <111> direction
(along the longest diagonal of the cubic diamond lattice). Therefore, whereas it
might be possible to scratch some diamonds with other materials, such as boron nitride,
the hardest diamonds can only be scratched by other diamonds. In particular, nanocrystalline
diamond aggregates were measured to be harder than any large single crystal diamond.
Those aggregates are produced by high-pressure high-temperature treatment of graphite
or fullerite (C60).
The hardness of diamonds contributes to its suitability as a gemstone. Because it
can only be scratched by other diamonds, it maintains its polish extremely well.
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 engagement
or wedding rings, which are often worn every day.
The hardest natural diamonds mostly originate from the Copeton and Bingara fields
located in 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 associated with the crystal growth form, which is single-stage
crystal growth. 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. It is possible to treat regular diamonds under a combination
of high pressure and high temperature to produce diamonds that are harder than the
diamonds used in hardness gauges.
Somewhat related to hardness is another mechanical property toughness, which is a
material's ability to resist breakage from forceful impact. The toughness of natural
diamond has been measured as 2.0 MPa·m1/2, and the critical stress intensity
factor is 3.4 MN·m-3/2. Those values are good compared to other gemstones,
but poor compared to most engineering materials. As with any material, the macroscopic
geometry of a diamond contributes to its resistance to breakage. Diamond has a cleavage
plane and is therefore more fragile in some orientations than others. Diamond cutters
use this attribute to cleave some stones, prior to faceting.
Electrical Conductivity
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. The conductivity and blue
color originate from the boron impurity. Boron substitutes for carbon atoms in the
diamond lattice, donating a hole into the valence band.
Substantial conductivity is commonly observed in nominally undoped diamond grown
by chemical vapor deposition. This conductivity is associated with hydrogen-related
species adsorbed at the surface, and it can be removed by annealing or other surface
treatments.
Color
Diamond has a wide bandgap of 5.5 eV corresponding to the deep ultraviolet wavelength
of 225 nanometers. This means pure diamond should transmit visible light and appear
as a clear colorless crystal. Colors in diamond originate from lattice defects and
impurities. The diamond crystal lattice is exceptionally strong and only atoms of
nitrogen, boron and hydrogen can be introduced into diamond during the growth at
significant concentrations (up to atomic percents). Transition metals Ni and Co,
which are commonly used for growth of synthetic diamond by the high-pressure high-temperature
techniques, have been detected in diamond as individual atoms, however the maximum
concentration is 0.01% for Ni and even much less for Co. Note however, that virtually
any element can be introduced in diamond by ion implantation.
Nitrogen is by far the most common impurity found in gem diamonds. Nitrogen is responsible
for the yellow and brown in diamonds. Boron is responsible for the gray blue colors.
Color in diamond has two additional sources: irradiation (usually by alpha particles),
that causes the color in green diamonds; and physical deformation of the diamond
crystal known as plastic deformation. Plastic deformation is the cause of color in
some brown[26] and perhaps pink and red diamonds. In order of rarity, colorless diamond,
by far the most common, is followed by yellow and brown, by far the most common colors,
then by blue, green, black, translucent white, pink, violet, orange, purple, and
the rarest, red. "Black", or Carbonado, diamonds are not truly black, but
rather contain numerous dark inclusions that give the gems their dark appearance.
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, known as a carbon flaw. The most common
impurity, nitrogen, causes a slight to intense yellow coloration depending upon the
type and concentration of nitrogen present. The Gemological Institute of America
(GIA) classifies low saturation yellow and brown diamonds as diamonds in the normal
color range, and applies a grading scale from "D" (colorless) to "Z"
(light yellow). Diamonds of a different color, such as blue, are called fancy colored
diamonds, and fall under a different grading scale.
Identification
Diamonds can be identified by their high thermal conductivity. Their high refractive
index is also indicative, but other materials have similar refractivity. Diamonds
do cut glass, but this does not positively identify a diamond because other materials,
such as quartz, also lie above glass on the Mohs scale and can also cut glass. Diamonds
can scratch other diamonds, but this can result in damage to one or both stones.
Hardness tests are infrequently used in practical gemology because of their potentially
destructive nature. The extreme hardness and high value of diamond means that gems
are typically polished slowly using painstaking traditional techniques and greater
attention to detail than is the case with most other gemstones;[30] these tend to
result in extremely flat, highly polished facets with exceptionally sharp facet edges.
Diamonds also possess an extremely high refractive index and fairly high dispersion.
Taken together, these factors affect the overall appearance of a polished diamond
and most diamantaires still rely upon skilled use of a loupe (magnifying glass) to
identify diamonds 'by eye'.
Synthetics
Synthetic diamonds are diamond crystals that are manufactured in a laboratory, as
opposed to natural diamonds which form naturally within the Earth. The gemological
and industrial uses of diamond have created a large demand for rough stones. This
demand has been satisfied in large part by synthetic diamonds, which have been manufactured
by various processes for more than half a century. However, in recent years it has
become possible to produce gem-quality synthetic diamonds of significant size.
The majority of commercially available synthetic diamonds are yellow in color and
produced by so called High Pressure High Temperature (HPHT) processes. The yellow
color is caused by nitrogen impurities. Other colors may also be reproduced such
as blue, green or pink, which are a result of the addition of boron or from irradiation
after synthesis.
Colorless gem cut from diamond grown by chemical vapor deposition
Another popular method of growing synthetic diamond is chemical vapor deposition
(CVD). The growth occurs under low pressure (below atmospheric pressure). It involves
feeding a mixture of gases (typically 1 to 99 methane to hydrogen) into a chamber
and splitting them to chemically active radicals in a plasma ignited by microwaves,
hot filament, arc discharge, welding torch or laser. This method is mostly used for
coatings, but can also produce single crystals several millimeters in size (see picture).
At present, the annual production of gem quality synthetic diamonds is only a few
thousand carats, whereas the total production of natural diamonds is around 120 million
carats. Despite this fact, a purchaser is more likely to encounter a synthetic when
looking for a fancy-colored diamond because nearly all synthetic diamonds are fancy-colored,
while only 0.01% of natural diamonds are fancy-colored.
Simulants
A diamond simulant is defined as a non-diamond material that is used to simulate
the appearance of a diamond. Diamond-simulant gems are often referred to as diamante.
The most familiar diamond simulant to most consumers is cubic zirconia. The popular
gemstone moissanite (silicon carbide) is often treated as a diamond simulant, although
it is a gemstone in its own right. While moissanite does look similar to diamond,
its main disadvantage as a diamond simulant is that cubic zirconia is far cheaper
and arguably equally convincing. Both cubic zirconia and moissanite are produced
synthetically.
Diamond Enhancement
Diamond enhancements are specific treatments performed on natural or synthetic diamonds
(usually those already cut and polished into a gem), which are designed to better
the gemological characteristics of the stone in one or more ways. These include laser
drilling to remove inclusions, application of sealants to fill cracks, treatments
to improve a white diamond's color grade, and treatments to give fancy color to a
white diamond.
Coatings are increasingly used to give a diamond simulant such as cubic zirconia
a more "diamond-like" appearance. One such substance is diamond-like carbon–an
amorphous carbonaceous material that has some physical properties similar to those
of the diamond. Advertising suggests that such a coating would transfer some of these
diamond-like properties to the coated stone, hence enhancing the diamond simulant.
However, modern techniques such as Raman Spectroscopy should easily identify such
as treatment.
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