Structural Properties
Formation
Diamonds are formed when carbon deposits are exposed to high pressure and temperature for prolonged periods. Deep within the earth, there are regions that are at a high enough temperature and pressure that the formation of diamonds is thermodynamically possible. Under the continental crust, diamonds form starting at depths of about 90 miles (150 kilometers), where pressure is roughly 5 gigapascals and the temperature is around 2,200 degrees Fahrenheit (1,200 degrees Celsius). Diamond formation under oceanic crust takes place at greater depths due to lower surface temperatures. This requires a higher pressure for diamond formation. Long periods of exposure to these high pressures and temperatures allow diamond crystals to grow larger.
Basic Properties
A diamond it is the hardest naturally occurring material. Diamond is one of several allotropes of carbon, the principle allotrope being graphite. Allotrope" or "Allotropy" specifically refers to the chemical bond structure between atoms. A diamond is a transparent, optically isotropic crystal with a high dispersion of 0.044, a refractive index of 2.42, and a specific gravity of 3.52.
Crystal Structure & Hardness
The chemical bond structure of diamond crystal is what gives this gemstone its hardness, toughness, and differentiates it from graphite. The name "diamond" (also known as adamant), is derived from the Greek adamas, "untameable", "invincible" or "unconquerable," referring to its incredible hardness. Diamonds score a 10 on the old Mohs scale of mineral hardness. A Type 2-A diamond has a hardness value of 167 GPa (±6) when scratched with an ultrahard fullerite tip, and a hardness value of 231 GPa (±5) when scratched with a diamond tip. The material "boron nitride", when in a form structurally identical to diamond, is nearly as hard as diamond. Additionally, a currently hypothetical material, beta carbon nitride, may also be as hard or harder.
Toughness
In the field materials science and metallurgy, toughness is the resistance to fracture of a material when stressed or impacted. Toughness is measured in units of "joules" per cubic meter (J/m3) in the SI system and "pound-force" per square inch in US units. Unlike hardness, which only denotes resistance to scratching, diamond's toughness is only fair to good. Particular cuts of diamonds are more prone to breakage, and thus may be uninsurable by reputable insurance companies. The culet is a facet designed exclusively to resist breakage. By contrast, very thin girdles are prone to much higher breakage.
Optical Properties
Optical Properties
The luster (or "lustre") of a diamond is described as adamantine, which means diamond-like. The word luster traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance. Some diamonds exhibit fluorescence of various colors under long wave ultra-violet light, but generally bluish-white, yellowish or greenish fluorescence under X-rays. Fluorescence is an optical phenomenon in which a molecule absorbs a high-energy photon, and re-emits it as a lower-energy (or longer-wavelength) photon. Some diamonds, particularly Canadian diamonds, show no fluorescence. 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 green at 504 nm, sometimes accompanied by two additional weak bands also in green.
Color & Composition
Diamonds occur in a wide variety of colors: colorless, white, blue, steel (grey), yellow, orange, pink, red, brown, green, and black. Colored diamonds contain certain impurities or structural defects that cause the coloration, while "pure" diamonds are transparent and colorless.
Type 1 diamonds have nitrogen atoms as the main impurity. If they are in clusters they do not affect the diamond's color (Type 1-A). If dispersed throughout the crystal they give the stone a yellow tint (Type 1-B). Typically a natural diamond crystal contains both Type 1-A and Type 1-B material. Synthetic diamonds containing nitrogen are Type 1-B.
Type 2 diamonds have very few nitrogen impurities. Type IIa diamond can be colored pink, red, or brown due to structural anomalies arising through plastic deformation. Type 2-B is the blue diamond containing scattered boron within the crystal matrix.
Experimentation in the late 18th century showed that diamonds were made of carbon. By igniting a diamond in an oxygen atmosphere, the end product of the combustion was carbonic acid gas (or carbon dioxide). Diamond had previously been shown to burn during Roman times although the reason was not understood at the time. 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.
Electromagnetic Properties
Electrical Properties
Diamond is a good electrical insulator, with the exception of most natural blue diamonds, which are actually semiconductors. Natural blue diamonds have been found to owe their color to an overabundance of hydrogen atoms. Most natural blue 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 out of diamonds. 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 7.4K.
Thermal Properties
Diamonds are a good conductor of heat, unlike most electrical insulators. Most natural blue diamonds contain boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductance. Purified synthetic diamond has the highest thermal conductivity (2000-2500 W/(m-K, five times greater than pure copper) of any known solid at room temperature. Because of a diamond's high thermal conductance, it is used in semiconductor manufacture to prevent silicon and other semiconducting materials from overheating.
Alternatives To Nature
Synthetic Diamonds
Both the gemological and industrial uses of diamond have created a large demand for raw stones and a large portion of this demand is now being met by "synthetic" or "man-made" diamonds. Although they are a simulation of the natural stone, they have very similar properties. This process has historically produced industrial-grade diamonds, but synthetic diamond producers have recently begun to penetrate the gem diamond market. Diamonds have been manufactured synthetically for over fifty years.
Materials which have similar gemological characteristics to diamond are known as diamond simulants. The most familiar diamond simulant to most consumers is cubic zirconium (CZ). A trained gemologists with appropriate equipment will be able to distinguish a natural diamond from a synthetic and simulant diamond, and will be able to identify any "enhancements" to the stone.
"Diamond Enhancements" are specific treatments, performed on cut and polished natural diamonds, which are designed to "improve" the gemological characteristics of the stone. These treatments 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.
Diamond Info Links
General Diamond Info
Diamond Chemistry & Artificial Diamonds
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