Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness and thermal conductivity (900–2320 W·m1·K1), as well as wide bandgap and high optical dispersion.
Above 1700 °C (1973 K / 3583 °F) in vacuum or oxygen-free atmosphere, diamond converts to graphite; in air, transformation starts at ~700 °C. Diamond's ignition point is 720 – 800 °C in oxygen and 850 – 1000 °C in air. Naturally occurring diamonds have a density ranging from 3.15–3.53 g/cm3, with pure diamond close to 3.52 g/cm3.
The chemical bonds that hold the carbon atoms in diamonds together are weaker than those in graphite. In diamonds, the bonds form an inflexible three-dimensional lattice, whereas in graphite, the atoms are tightly bonded into sheets, which can slide easily over one another, making the overall structure weaker. In a diamond, each carbon atom is surrounded by neighboring four carbon atoms forming a tetrhedral shaped unit.
Diamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. Diamond is an optically isotropic crystal that is transparent to opaque. Owing to its strong covalent bonding, diamond is the hardest naturally occurring material known. Yet, due to important structural weaknesses, diamond's toughness is only fair to good. The precise tensile strength of diamond is unknown, however strength up to 60 GPa has been observed, and it could be as high as 90–225 GPa depending on the crystal orientation.
 The anisotropy of diamond hardness is carefully considered during diamond cutting. Diamond has a high refractive index (2.417) and moderate dispersion (0.044) properties which give cut diamonds their brilliance. Scientists classify diamonds into four main types according to 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.
Material properties of diamond. Hardness and crystal structure Molar volume vs. pressure at room temperature.
Visualisation of a diamond cubic unit cell: 1. Components of a unit cell, 2. One unit cell, 3. A lattice of 3×3×3 unit cells Because of its great hardness and strong molecular bonding, a cut diamond's facets and facet edges appear the flattest and sharpest. Hardness and crystal structure. Toughness and pressure resistance.
Electrical properties. Thermal conductivity. Thermal stability. Surface properties. Chemical stability. Identification. Crystallographic defects in diamond. Synthetic diamonds of various colors grown by the high-pressure high-temperature technique, the diamond size is ~2 mm.
Covalent superconductor. Covalent superconductors are superconducting materials where the atoms are linked by covalent bonds.
The first such material was synthetic diamond grown by the high-pressure high-temperature (HPHT) method. The discovery had no practical importance, but surprised most scientists as superconductivity had not been observed in covalent semiconductors, including diamond and silicon. Diamond Superconductivity in diamond was achieved through heavy p-type doping by boron such that the individual doping atoms started interacting and formed an "impurity band".
The superconductivity was of type-II with the critical temperature Tc = 4 K and critical magnetic field Hc = 4 T. Later, Tc ~ 11K has been achieved in homoepitaxial CVD films. Silicon Silicon carbide Superconductivity in SiC was achieved by heavy doping with boron or aluminum. Both the cubic (3C-SiC) and hexagonal (6H-SiC) phases are superconducting and show a very similar Tc of 1.5 K. Diamond color. Possible colors Main article: Diamond type Diamonds occur in a variety of colors—steel gray, white, blue, yellow, orange, red, green, pink to purple, brown, and black. Colored diamonds contain interstitial impurities or structural defects that cause the coloration, whilst pure diamonds are perfectly transparent and colorless.
Diamonds are scientifically classed into two main types and several subtypes, according to the nature of impurities present and how these impurities affect light absorption: Type I diamonds have nitrogen atoms as the main impurity, commonly at a concentration of 0.1%. If the nitrogen atoms are in pairs they do not affect the diamond's color; these are Type IaA. Type II diamonds have no measurable nitrogen impurities. Grading white diamonds The majority of diamonds that are mined are in a range of pale yellow or brown color that is termed the normal color range.