Material properties of diamond

Diamond
An octahedral diamond crystal in matrix
General
Category	Native Nonmetal, Mineral
Formula (repeating unit)	Carbon (C)
Identification
Colour	Most often colorless to yellow or brown. Rarely pink, orange, green or blue.
Crystal habit	Octahedral, spherical or massive
Crystal system	Isometric
Cleavage	Octahedral; perfect and easy
Fracture	Conchoidal
Mohs scale hardness	10
Luster	Adamantine to greasy
Streak	None
Diaphaneity	Clear to not
Specific gravity	3.516–3.525
Refractive index	2.417
Pleochroism	None
Fusibility	Burns above 800°C, on heating over 2000 Celsius decomposes in solid state without melting.
Solubility	Resistant to acids, but dissolves irreversibly in hot steel
Major varieties
Ballas	Spherical, radial structure, cryptocrystalline, opaque black
Bort	Poorly formed, cryptocrystalline, shapeless, translucent
Carbonado	Massive, microcrystalline, opaque black

Diamond is transparent to opaque, optically isotropic, 3D-crystalline carbon. It is the hardest naturally occurring material known—owing to its strong covalent bonding—yet its toughness is only fair to good due to important structural weaknesses. The precise tensile strength of diamond is unknown. However, strength up to 60 GPa has been observed, and its theoretical intrinsic strength has been calculated to be between 90 and 225 GPa, depending on the crystal orientation. Diamond has a high refractive index (2.417) and moderate dispersion (0.044), properties which are considered carefully during diamond cutting and which (together with their hardness) give cut diamonds their brilliance and fire. Scientists classify diamonds into two main types and several subtypes, depending on the nature of crystallographic defects present. Trace impurities substitutionally replacing carbon atoms in a diamond's crystal lattice, and in some cases structural defects, are responsible for the wide range of colors seen in diamond. Most diamonds are electrical insulators but extremely efficient thermal conductors. The specific gravity of single-crystal diamond (3.52) is fairly constant. Contrary to a common misconception, diamond is not the most stable form of solid carbon; graphite has that distinction.

Hardness and crystal structure

Known to the ancient Greeks as adamas ("tame'sles" or "bridleless") and sometimes called adamant, diamond 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. Furthermore, it has been shown 1 2 that ultrahard fullerite (C⁶⁰) (not to be confused with P-SWNT fullerite) when testing diamond hardness with a scanning force microscope can scratch diamond. In turn, using more accurate measurements, these values are now known for diamond hardness. A Type IIa diamond (111) has a hardness value of 167 GPa (±6) when scratched with an ultrahard fullerite tip, while an ultrahard fullerite sample has a value of 310 GPa when tested with a fullerite tip. However, the test only works properly with a tip made of harder material than the sample being tested. This means that the true value for ultrahard fullerite is likely somewhat lower than 310 GPa.

Cubic diamonds have a perfect and easy octahedral cleavage, which means that they have four planes—directions following the faces of the octahedron where there are fewer bonds and therefore points of structural weakness—along which diamond can easily split (following a blunt impact), leaving smooth surfaces. Similarly, diamond's hardness is markedly directional: the hardest direction is the diagonal on the cube face, 100 times harder than the softest direction, which is the dodecahedral plane. The octahedral plane, followed by the axial directions on the cube plane, are intermediate between the two extremes. The diamond cutting process relies heavily on this directional hardness, as without it a diamond would be nearly impossible to fashion. Cleavage also plays a helpful role, especially in large stones where the cutter wishes to remove flawed material or to produce more than one stone from the same piece of rough.

Diamonds typically crystallize in the cubic crystal system ( space group ) and consist of tetrahedrally, covalently bonded carbon atoms. A second form called lonsdaleite with hexagonal symmetry is also found, but it is extremely rare and is believed to form only when meteoric graphite falls to Earth. The local environment of each atom is identical in the two structures. In terms of crystal habit, diamonds occur most often as euhedral (well-formed) or rounded octahedra and twinned, flattened octahedra known as macles (with a triangular outline). Other forms include dodecahedra and (rarely) cubes. There is some evidence that interstitial nitrogen impurities play an important role in the formation of euhedral crystals—the largest diamonds found, such as the Cullinan Diamond, have been shapeless or massive. These diamonds are Type II and therefore contain little if any nitrogen (see Composition and colour).

The faces of diamond octahedrons are highly lustrous due to their hardness; growth defects in the form of trigons or etch pits are often present on the faces, the former being triangular pits whose points are aligned with the faces of the octahedron. A diamond's fracture may be step-like, conchoidal (shell-like, similar to glass) or irregular. Diamonds which are nearly round due to the stepping tendency of octahedrons are commonly found coated in nyf, a gum-like skin; the combination of stepped faces, growth defects, and nyf produces a "scaly" or corrugated appearance, and such diamonds are termed crinkles. A significant number of diamonds crystallize anhedrally: that is, their forms are so distorted that few crystal faces are discernible. Some diamonds found in Brazil and the Democratic Republic of the Congo are cryptocrystalline and occur as opaque, darkly colored, spherical, radial masses of tiny crystals; these are known as ballas and are important to industry as they lack the cleavage planes of single-crystal diamond. Carbonado is a similar opaque microcrystalline form which occurs in shapeless masses. Like ballas diamond, carbonado lacks cleavage and its specific gravity varies widely, from 2.9–3.5. Bort diamonds, found in Brazil, Venezuela, and Guyana, are the most common type of industrial-grade diamond, also cryptocrystalline or otherwise poorly crystallized, but possessing cleavage, translucency, and lighter colors.

Due to its great hardness and strong molecular bonding, a cut diamond's facets and facet edges are observably the flattest and sharpest. A curious side effect of diamond's surface perfection is hydrophobia combined with lipophilia. The former property means a drop of water placed on a diamond will form a coherent droplet, whereas in most other minerals the water would spread out to cover the surface. Similarly, diamond is unusually lipophilic, meaning grease and oil readily collect on a diamond's surface. Whereas on other minerals oil would form coherent drops, on a diamond the oil would spread. This property is exploited in the use of so-called "grease pens," which apply a line of grease to the surface of a suspect diamond simulant.

Diamond is so strong because of the shape the carbon atoms make. It's a very strong 3D shape, each carbon atom having four joined to it with covalent bonds.

Toughness

Unlike hardness, which only denotes resistance to scratching, diamond's toughness or tenacity is only fair to good. Toughness relates to the ability to resist breakage from falls or impacts: due to diamond's perfect and easy cleavage, it is vulnerable to breakage. A diamond will shatter if hit with an ordinary hammer.

Ballas and carbonado diamond are exceptional, as they are polycrystalline and therefore much tougher than single-crystal diamond; they are used for deep-drilling bits and other demanding industrial applications. Particular cuts of diamonds are more prone to breakage—such as marquis or other cuts featuring tapered points—and thus may be uninsurable by reputable insurance companies. The culet is a facet (parallel to the table) given to the pavilion of cut diamonds designed specifically to reduce the likelihood of breakage or splintering. Extremely thin, or very thin girdles are also prone to much higher breakage.

Solid foreign crystals are commonly present in diamond—these and other inclusions, such as internal fractures or "feathers"—can compromise the structural integrity of a diamond. Cut diamonds that have been enhanced to improve their clarity via glass infilling of fractures or cavities are especially fragile, as the glass will not stand up to ultrasonic cleaning or the rigors of the jeweler's torch. Fracture-filled diamonds may shatter if treated improperly.

Optical properties

The lustre of a diamond is described as 'adamantine', which simply means diamond-like. It is the highest luster possible bar that of metal (metallic), and is due to diamond's superlative hardness. Reflections on a properly cut diamond's facets are undistorted, due to their flatness. The refractive index of diamond (as measured via sodium light, 589.3 nm) is 2.417; because it is cubic in structure, diamond is also isotropic. Its high dispersion of 0.044 (B-G interval) manifests in the perceptible fire of cut diamonds. This fire—flashes of prismatic colors seen in transparent stones—is perhaps diamond's most important optical property from a jewelry perspective. The prominence or amount of fire seen in a stone is heavily influenced by the choice of diamond cut and its associated proportions (particularly crown height), although the body colour of fancy diamonds may hide their fire to some degree.

Some diamonds exhibit fluorescence of various colors and intensities under long wave (LW) ultra-violet light (365 nm): Cape series stones (Type Ia; see composition and colour) usually fluoresce blue, and these stones may also phosphoresce yellow. (This is a unique property among gemstones). Other LW fluorescence colors possible are green (usually in brown stones), yellow, mauve, or red (Type IIb). In natural diamonds there is typically little if any response to shortwave (SW) ultraviolet, but the reverse is true of synthetics. Some natural Type IIb diamonds may phosphoresce blue after exposure to SW ultraviolet. In naturals, fluorescence under X-rays is generally bluish-white, yellowish or greenish. Some diamonds, particularly Canadian diamonds, show no fluorescence.

Cape series diamonds have a visible absorption spectrum (as seen through a direct-vision spectroscope) consisting of a fine line in the violet at 415.5 nm—however, this line is often invisible until the diamond has been cooled to very low temperatures. Associated with this are weaker lines at 478 nm (often only this line is visible), 465 nm, 452 nm, 435 nm, and 423 nm. Other stones show additional bands: brown, green, or yellow diamonds show a band in the green at 504 nm, sometimes accompanied by two additional weak bands at 537 nm and 495 nm. Type IIb diamonds may absorb in the far red, but otherwise show no observable visible absorption spectrum.

Gemological laboratories make use of spectrophotometer machines that can distinguish natural, artificial, and colour- enhanced diamonds. The spectrophotometers analyze the infrared, visible, and ultraviolet absorption spectrums of diamonds cooled with liquid nitrogen to detect tell-tale absorption lines that are not normally discernible.

Electrical properties

Except for most natural blue diamonds—which are semiconductors due to substitutional boron impurities replacing carbon atoms—diamond is a good electrical insulator. Natural blue diamonds recently recovered from the Argyle diamond mine in Australia have been found to owe their colour to an overabundance of hydrogen atoms: these diamonds are not semiconductors. 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 from 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. In 2005 reports came out that the National Institute of Advanced Industrial Science and Technology (AIST) in Japan created an n-type diamond semiconductor , and a Light Emitting Diode ( LED) producing 235 nm UV light.

In April of 2004 Nature reported that below the superconducting transition temperature 4 K, boron-doped diamond synthesized at high temperature and high pressure is a bulk, type-II superconductor . In October of 2004 superconductivity was found to occur in heavily boron-doped microwave plasma-assisted chemical vapor deposition (MPCVD) diamond below the superconducting transition temperature of 7.4 K .

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 conductance. .999-¹²C monocrystalline synthetic diamond has the highest thermal conductivity of any known solid at room temperature: 2000–2500 W·m/m²·K (200–250 W·mm/cm²·K), five times more than copper. Because diamond has such high thermal conductance it is already used in semiconductor manufacture to prevent silicon and other semiconducting materials from overheating. At lower temperatures conductivity becomes even better as its Fermi electrons can match the phononic normal transport mode near the Debye point, and transport heat more swiftly, to overcome the drop of specific heat with the fewer quantal microstates, to reach 41,000 W·m/m²·K at 104 K. The same diamond at .99999-¹²C is predicted to 200,000 W·m/m²·K (20 kW·mm/cm²·K).

Diamond's thermal conductivity is made use of by jewellers and gemologists who may employ an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. This test takes about 2–3 seconds. However, older probes will be fooled by moissanite, an imitation of diamond introduced in 1998 which has a similar thermal conductivity.

Being a form of carbon, they can burn in the presence of oxygen if heated over 800 °C (1500 °F). In absence of oxygen they can stand higher temperatures, but will convert to graphite eventually.

Composition, colour, and stability

Diamonds occur in a restricted variety of colors—black, brown, yellow, grey, white, blue, orange, purple to pink, red, and chartreuse. Colored diamonds contain crystallographic defects, including substitutional impurities and structural defects, that cause the coloration. Theoretically, pure diamonds would be transparent and colorless. Diamonds are scientifically classed into two main types and several subtypes, according to the nature of defects present and how they affect light absorption:

Type I diamond has nitrogen (N) atoms as the main impurity, at a concentration of 0.1 percent. If the N atoms are in pairs they do not affect the diamond's colour; these are Type IaA. If the N atoms are in large even-numbered aggregates they impart a yellow to brown tint (Type IaB). About 98 percent of gem diamonds are type Ia, and most of these are a mixture of IaA and IaB material: these diamonds belong to the Cape series, named after the diamond-rich region formerly known as Cape Province in South Africa, whose deposits are largely Type Ia. If the N atoms are dispersed throughout the crystal in isolated sites (not paired or grouped), they give the stone an intense yellow or occasionally brown tint (Type Ib); the rare canary diamonds belong to this type, which represents only one permille of known natural diamonds. Synthetic diamond containing nitrogen is Type Ib. Type I diamonds absorb in both the infrared and ultraviolet region, from 320 nm. They also have a characteristic fluorescence and visible absorption spectrum (see Optical properties).

Type II diamonds have very few if any nitrogen impurities. Type IIa diamond can be colored pink, red, or brown due to structural anomalies arising through plastic deformation during crystal growth—these diamonds are rare (1.8 percent of gem diamonds), but constitute a large percentage of Australian production. Type IIb diamonds, which account for 0.1 percent of gem diamonds, are usually a steely blue or grey due to scattered boron within the crystal matrix; these diamonds are also semiconductors, unlike other diamond types (see Electrical properties). However, an overabundance of hydrogen can also impart a blue colour; these are not necessarily Type IIb. Type II diamonds absorb in a different region of the infrared, and transmit in the ultraviolet below 225 nm, unlike Type I diamonds. They also have differing fluorescence characteristics, but no discernible visible absorption spectrum.

Certain diamond enhancement techniques are commonly used to artificially produce an array of colors, including blue, green, yellow, red, and black. Colour enhancement techniques usually involve irradiation, including proton and deuteron bombardment via cyclotrons; neutron bombardment via the piles of nuclear reactors; and electron bombardment via Van de Graaff generators. These high-energy particles physically alter the diamond's crystal lattice, knocking carbon atoms out of place and producing color centers. The depth of colour penetration depends on the technique and its duration, and in some cases the diamond may be left radioactive to some degree.

It should be noted that some irradiated diamonds are completely natural—one famous example is the Dresden Green Diamond. In these natural stones the color is imparted by "radiation burns" in the form of small patches, usually only skin deep. Additionally, Type IIa diamonds can have their structural deformations "repaired" via a high-temperature, high-pressure (HTHP) process, removing much or all of the diamond's colour.

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 / mol). Diamonds had previously been shown to burn during Roman times.

So, despite De Beers' 1948 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.