Material properties

A diamond is a transparent crystal of tetrahedrally bonded carbon atoms (sp3) that crystallizes into the face centered cubic diamond lattice structure. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness, its high dispersion index, and extremely high thermal conductivity (900 – 2320 W/m K). Above 1700 °C (1973 K / 3583 °F), diamond is converted to graphite.[9] Naturally occurring diamonds have a density ranging from 3.15 to 3.53 g/cm³, with very pure diamond typically extremely close to 3.52 g/cm³.

Hardness

Diamond is the hardest natural material known, where hardness is defined as resistance to scratching.[10] Diamond has a hardness of 10 (hardest) on Mohs scale of mineral hardness.[11] Diamond's hardness has been known since antiquity, and is the source of its name.

The hardest diamonds in the world are from the Copeton and Bingara fields located in the New England area in New South Wales, Australia. They were called can-ni-fare (cannot be cut) by the cutters in Antwerp when they started to arrive in quantity from Australia in the 1870s. 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.[12]

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.

Industrial use of diamonds has historically been associated with their hardness; this property makes diamond the ideal material for cutting and grinding tools. 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, and the use of diamond powder as an abrasive. Less expensive industrial-grade diamonds, known as bort, with more flaws and poorer color than gems, are used for such purposes.

Diamond is not suitable for machining ferrous alloys at high speeds as carbon is soluble in iron at the high temperatures created by high-speed machining, leading to greatly increased wear on diamond tools when compared to alternatives.

These substances can scratch diamond:

  * Some diamonds are harder than others.
  * boron nitride
  * Borazon - a boron nitride allotrope
  * Rhenium diboride
  * Hexagonal form of diamond called lonsdaleite, is theoretically predicted to be 58% stronger than diamond. [13]
  * Aggregated diamond nanorods, a material produced by high-pressure high-temperature treatment of fullerite (C60).

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.[11] 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.[14] This conductivity is associated with hydrogen-related species adsorbed at the surface, and it can be removed by annealing or other surface treatments.

Toughness

Toughness relates to a material's ability to resist breakage from forceful impact. The toughness of natural diamond has been measured as 3.4 MN m-3/2,[15] which is 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.

Color
Main article: Diamond color
Brown colored diamonds at the National Museum of Natural History

Gem quality diamond may be colorless or occur in any hue including the non-spectral hues of gray, brown and black. Diamond is the only gemstone composed of a single element, carbon. The diamond crystal lattice is exceptionally strong and only atoms of nitrogen, boron, hydrogen, phosphorus and maybe beryllium can be introduced into diamond during the growth at significant concentrations. 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[16] and even much less for Co. Note however, that virtually any element can be introduced in diamond by ion implantation.

Nitrogen is the smallest and 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[17] and perhaps pink and red diamonds.[18] 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.[11] "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.[11] 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.

In 2008, the Wittelsbach Diamond, a 35.56 carats (7.11 g) blue diamond once belonging to the King of Spain, fetched over $24M US at a Christie's auction. The blue hue was a result of trace amounts of boron in the stone's crystal structure.[19]

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 other materials, such as quartz, also lie above glass on Mohs scale. Diamonds easily scratch other diamonds, but this damages both diamonds.

Natural history

The formation of natural diamond requires very specific conditions. Diamond formation requires exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars,[20] but at a comparatively low temperature range between approximately 1650–2370 °F (900–1300 °C).[20] These conditions are known to be met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.