Material properties

Diamond is a transparent crystal of pure carbon consisting of tetrahedrally bonded carbon atoms. Humans have been able to adapt diamonds for many uses because of the material's exceptional physical characteristics. Most notable among these properties are the extreme hardness of diamond, its high dispersion index, and high thermal conductivity.

• Mechanical properties

  • Crystal structure

    Diamonds typically crystallize in the cubic crystal system and consist of tetrahedrally bonded carbon atoms. A colorless, grey or black diamond with a tiny radial structure is a spherulite.
    
    The tetrahedral arrangement of atoms in a diamond crystal is the source of many of diamond's properties; graphite, another allotrope of carbon, has a rhombohedral crystal structure and as a result shows dramatically different physical characteristics contrary to diamond, graphite is a very soft, dark grey, opaque mineral.
     

  • Hardness

    The diamond crystal bond structure gives the gem its hardness and differentiates it from graphite.Diamond is the hardest known naturally occurring material, scoring 10 on the relative Mohs scale of mineral hardness and having an absolute hardness value of between 167 and 231 gigapascals in various tests. Diamond's hardness has been known since antiquity, and is the source of its name.
    
    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, keeping its luster over long periods of time.
    
    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 an engagement ring or wedding ring, which are often worn everyday.
     

  • Toughness

    Unlike hardness, which only denotes resistance to scratching, diamond's toughness is only fair to good. Toughness relates to a material's ability to resist breakage from forceful impact. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamonds cut into certain particular shapes are therefore more prone to breakage than others.
     

  • Color

    Diamonds occur in a variety of transparent hues colorless, white, steel, blue, yellow, orange, red, green, pink, brown or colored black. Diamonds with a detectable hue to them are known as colored diamonds. 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. The most common impurity, nitrogen, causes a yellowish or brownish tinge.
     

  • Thermodynamic stability

    At surface air pressure (one atmosphere), diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (?G = -2.99 kJ / mol). Diamonds will burn at approximately 800 degrees Celsius, providing that enough oxygen is available.
    
    However, owing to a very large kinetic energy barrier, diamonds are metastable; under normal conditions, it would take an extremely long time (possibly more than the age of the Universe) for diamond to decay into graphite.
     

  • Optical properties

    Diamonds exhibit a high dispersion of visible light. This strong ability to split white light into its component colors is an important aspect of diamond's attraction as a gemstone, giving it impressive prismatic action that results in so-called fire in a well-cut stone. The luster of a diamond, a characterization of how light interacts with the surface of a crystal, is brilliant and is described as adamantine, which simply means diamond-like. This is owed to their high refractive index of 2.417 (at 589.3 nm), which causes total internal reflection to occur.

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  Natural History

• Formation

  Diamond is formed by prolonged exposure of carbon bearing materials to high pressure and temperature. On Earth, the formation of diamonds is possible because there are regions deep within the Earth that are at a high enough pressure and temperature that the formation of diamonds is thermodynamically favorable . Under continental crust, diamonds form starting at depths of about 150 kilometers (90 miles), where pressure is roughly 5 gigapascals and the temperature is around 1200 degrees Celsius (2200 degrees Fahrenheit). Diamond formation under oceanic crust takes place at greater depths because of higher temperatures, which require higher pressure for diamond formation. Long periods of exposure to these high pressures and temperatures allow diamond crystals to grow larger.
  
  Diamonds that have come to the Earth's surface are generally very old, ranging from under 1 billion to 3.3 billion years old.
  
  Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles. As diamond's crystal structure has a cubic arrangement of the atoms, they have many facets that belong to a cube, octahedron, rhombicosidodecahedron, tetrakis hexahedron or disdyakis dodecahedron. The crystals can have rounded off and unexpressive edges and can be elongated. Sometimes they are found grown together or form double "twinned" crystals grown together at the surfaces of the octahedron. This is all due to the conditions in which they form.
   

• Surfacing

  Schematic diagram of a volcanic pipeDiamond-bearing rock is forced close to the surface through deep-origin volcanic eruptions. The magma for such a volcano must originate at a depth where diamonds can be formed, 90 miles (150 km) deep or more (three times or more the depth of source magma for most volcanoes); this is a relatively rare occurrence. Below these typically small surface volcanic craters are formations known as volcanic pipes, which contain material that was pushed toward the surface of the earth by volcanic action, but did not erupt before the volcanic activity ceased.
  
  Diamond-bearing volcanic pipes are most commonly found in the oldest regions of continental crust, which relates to the fact that these areas are the coolest portions of the earth's crust, and therefore diamonds can form at the shallowest depths.
  
  Once diamonds have been forced tlite or lamproite matrix, accumulate because of water or weather action.
  
  Diamonds can also be brought to the surface through certain processes which may occur when two continental plates collide forcefully, although this phenomenon is less understood and currently assumed to be uncommon.
  Over time, especially since around 1900, experts in the field of gemology have developed methods of characterizing diamonds and other gemstones based on the characteristics most important to their value as a gem. Four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds: these are carat, clarity, color, and cut.
  
  Other characteristics not described by the four Cs can and do influence the value or appearance of a gem diamond. These characteristics include physical characteristics such as the presence of fluorescenc. Cleanliness also dramatically affects a diamond's beauty.

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