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 (900șC to 1400șC) and pressure (5 to 6 GPa) 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.
Diamonds are mined in alluvial mining operations when they are not located along a "kimberlite pipe". Kimberlite is an ultrapotassic, ultramafic, igneous rock composed of garnet, olivine, phlogopite, and pyroxene with a variety of other trace minerals. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes.
Basic Properties
A diamond it is the hardest naturally occurring material with a relative hardness of 10 on the Mohs scale. 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 (10 on the 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.
Crystal Habit
Diamonds have a characteristic crystalline structure. This means that crystals usually "grow" in an orderly and symmetrical arrangement. The natural form, or habit, of a diamond is octahedral (above). In nature perfect crystals are rare. The external shape of the crystal does not always reflect the internal arrangement of atoms. When stones have irregular external shapes or arrangements of crystal faces, they are termed "subhedral" or "anhedral." Trace impurities, crystal twinning, and growth conditions (heat, pressure and space) can also effect the final shape of a formed crystal.
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.
Although diamond is the "hardest" (most scratch resistant) mineral, with a Mohs rating of 10, its toughness rating is only good. Sapphire has a hardness rating of 9, meaning that a diamond is 4 times "harder" than sapphire, yet sapphire has a toughness rating of excellent. Hematite has a hardness of only 5.5 to 6.5 but its toughness rating is also excellent.
Optical Properties
Fluorescence
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.
Type I & Type II Diamonds
Up to 99% of all natural diamonds are classified as Type I and contain nitrogen atoms as an impurity, replacing carbon atoms within the lattice structure. Nitrogen impurities in Type I diamonds are evenly dispersed throughout the stone, absorbing some of the blue spectrum, thereby making the diamond appear yellow. There are two subcategories (a and b) within each diamond 'type' that are based on a stone's electrical conductivity.
Diamonds that have formed under extremely high pressure for longer periods have a lower nitrogen content, permitting the passage of blue light and making the stone have a 'colorless' (D) appearance. Type II diamonds do not contain detectable nitrogen, thereby allowing the passage of short-wave ultra-violet (SWUV) light through the stone. Natural blue Type II diamonds containing boron impurities and are good conductors of electricity and are classified as Type IIb diamonds, and diamonds that lack boron impurities are classified as Type IIa. Type IIa diamonds have a near-perfect crystal structure making them highly transparent and colorless, with high thermal conductivity. Type IIa diamonds are very rare and some of the finest historical stones such as the Cullinan and Koh-i-Noor are both Type IIa diamonds.
Refraction & Coloration
Diamond is singly refractive with a refractive index of 2.417. Diamonds exhibit Pseudochromatic Coloration giving the appearance of "color" that is not caused by any actual color in the mineral, but from varying optics effects created by spectral dispersion (Fire) and refraction.
Diamonds can also exhibit Allochromatic Coloration that is caused by chromophores from the nitrogen trace impurities found within crystalline structure. It is Nitrogen that produces the yellow color in diamond.
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 in experiments conducted in the ancient Roman period 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
Insulators or Semiconductors
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.
Improvements To Nature
Enhanced Diamonds
"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. A trained gemologist will be able to identify any "enhancements" to the stone.
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General Diamond Info
Diamond Chemistry & Artificial Diamonds
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