Diamond is the hardest natural material. The Mohs hardness scale, on which diamond is a '10' and corundum (sapphire) is a '9', doesn't adequately attest to this incredible hardness, as diamond is exponentially harder than corundum. Diamond is also the least compressible and stiffest substance. It is an exceptional thermal conductor - 4 times better than copper - which gives significance to diamonds being called 'ice'. Diamond has an extremely low thermal expansion, is chemically inert with respect to most acids and alkalis, is transparent from the far infrared through the deep ultraviolet, and is one of only a few materials with a negative work function (electron affinity). One consequence of the negative electron affinity is that diamonds repel water, but readily accept hydrocarbons such as wax or grease. Diamonds do not conduct electricity well, although some are semiconductors. Diamond can burn if subjected to a high temperature in the presence of oxygen. Diamond has a high specific gravity; it is amazingly dense given the low atomic weight of carbon. The brilliance and fire of a diamond are due to its high dispersion and high refractive index. Diamond has the highest reflectance and index of refraction of any transparent substances. Diamond gemstones are commonly clear or pale blue, but colored diamonds, called 'fancies', have been found in all the colors of the rainbow. Boron, which lends a bluish color, and nitrogen, which adds a yellow cast, are common trace impurities. Two volcanic rocks that may contain diamonds are kimberlite and lamproite. Diamond crystals frequently contain inclusions of other minerals, such as garnet or chromite. Many diamonds fluoresce blue to violet, sometimes strongly enough to be seen in daylight. Some blue-fluorescing diamonds phosphoresce yellow (glow in the dark in an afterglow reaction).

Type of Diamonds

•Natural Diamonds

Natural diamonds are classified by the type and quantity of impurities found within them.

•Type Ia - This is the most common type of natural diamond, containing up to 0.3% nitrogen.

•Type Ib - Very few natural diamonds are this type (~0.1%), but nearly all synthetic industrial diamonds are. Type Ib diamonds contain up to 500 ppm nitrogen.

•Type IIa - This type is very rare in nature. Type IIa diamonds contain so little nitrogen that it isn't readily detected using infrared or ultraviolet absorption methods.

•Type IIb - This type is also very rare in nature. Type IIb diamonds contain so little nitrogen (even lower than type IIa) that the crystal is a p-type semiconductor.

•Synthetic Industrial Diamonds

Synthetic industrial diamonds are produced the process of High Pressure High Temperature Synthesis (HPHT). In HPHT synthesis, graphite and a metallic catalyst are placed in a hydraulic press under high temperatures and pressures. Over the period of a few hours the graphite converts to diamond. The resulting diamonds are usually a few millimeters in size and too flawed for use as gemstones, but they are extremely useful as edges on cutting tools and drill-bits and for being compressed to generate very high pressures. (Interesting side note: Although used to cut, grind, and polish many materials, diamonds aren't used to machine alloys of iron because the diamond abrades very quickly, due to a high-temperature reaction between iron and carbon.)

•Thin Film Diamonds

A process called Chemical Vapor Deposition (CVD) may be used to deposit thin films of polycrystalline diamond. CVD technology makes it possible to put 'zero-wear' coatings on machine parts, use diamond coatings to draw the heat away from electronic components, fashion windows that are transparent over a broad wavelength range, and take advantage of other properties of diamonds.

Additional Reading

•Diamond - Molecule of the Month - This site covers the properties of diamonds, the differences between diamond and graphite, classification of natural diamonds, manufacture of synthetic industrial diamond, and chemical vapor deposition of polycrystalline diamond.

•Links to Groups Studying CVD Diamond - The Bristol University chemical vapor deposition diamond group maintains this list of other groups engaged in similar research.

•The Chemistry of Carbon - This article provides an overview of basic chemistry associated with carbon, emphasizing the crystallographic difference between graphite and diamond.

•The Nature of Diamonds - The American Museum of Natural History offers this exceptional site, which includes a description of diamonds, discussion of origins and history, mining and distribution information, use in industry and technology, use in jewelry and gems, and bibliography.