Diamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. It is a crystal that is transparent to opaque and which is generally isotropic (no or very weak birefringence). Diamond is the hardest naturally occurring material known. Yet, due to important structural brittleness, bulk diamond's toughness is only fair to good. The precise tensile strength of bulk diamond is little known; however, compressive strength up to has been observed, and it could be as high as in the form of micro/nanometer-sized wires or needles (~ in diameter, micrometers long), with a corresponding maximum tensile elastic strain in excess of 9%. 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 that 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 structure, and in some cases structural defects, are responsible for the wide range of colors seen in diamond. Most diamonds are electrical insulators and extremely efficient thermal conductors. Unlike many other minerals, the specific gravity of diamond crystals (3.52) has rather small variation from diamond to diamond.

Hardness and crystal structure

Known to the ancient Greeks as (, 'proper, unalterable, unbreakable') and sometimes called adamant, diamond is the hardest known naturally occurring material, and serves as the definition of 10 on the Mohs scale of mineral hardness. Diamond is extremely strong owing to its crystal structure, known as diamond cubic, in which each carbon atom has four neighbors covalently bonded to it. Bulk cubic boron nitride (c-BN) is nearly as hard as diamond. Diamond reacts with some materials, such as steel, and c-BN wears less when cutting or abrading such material. (Its zincblende structure is like the diamond cubic structure, but with alternating types of atoms.) A currently hypothetical material, beta carbon nitride (β-), may also be as hard or harder in one form. It has been shown that some diamond aggregates having nanometer grain size are harder and tougher than conventional large diamond crystals, thus they perform better as abrasive material. The toughness of natural diamond has been measured as , which is good compared to other gemstones like aquamarine (blue colored), 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. and high electric breakdown field at room temperature are also important characteristics of diamond. Those characteristics allow single crystalline diamond to be one of the promising materials for semiconductors. A wide bandgap is advantageous in semiconductors because it allows them to maintain high resistivity even at high temperature, important for high power applications. Semiconductors whose carrier mobilities are high such as diamond are easier to utilize in industry because they do not need high input voltage. High breakdown voltage avoids a huge current suddenly occurring at typical input voltages.

Most natural blue diamonds are an exception and are semiconductors due to substitutional boron impurities replacing carbon atoms. Natural blue or blue-gray diamonds, common for the Argyle diamond mine in Australia, are rich in hydrogen; these diamonds are not semiconductors and it is unclear whether hydrogen is actually responsible for their blue-gray color. In January 2024, a Japanese research team fabricated a MOSFET using phosphorus-doped n-type diamond, which would have superior characteristics to silicon-based technology in high-temperature, high-frequency or high-electron mobility applications. FETs with SiN dielectric layers, and SC-FETs have been made.

In April 2004, research published in the journal Nature reported that below , synthetic boron-doped diamond is a bulk superconductor.

Thermal stability

thumb|Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.

If heated over in air, diamond, being a form of carbon, oxidizes and its surface blackens, but the surface can be restored by re-polishing.

Further reading

  • Pagel-Theisen, Verena. (2001). Diamond grading ABC: The manual (9th ed.), pp. 84–85. Rubin & Son n.v.; Antwerp, Belgium.
  • Webster, Robert, and Jobbins, E. Allan (Ed.). (1998). Gemmologist's compendium, p. 21, 25, 31. St Edmundsbury Press Ltd, Bury St Edmunds.
  • Properties of diamond
  • Properties of diamond (S. Sque, PhD thesis, 2005, University of Exeter, UK)