Diamond is a transparent, optically isotropic crystal with a refractive index of 2.417, a high dispersion of 0.044, and a specific gravity of 3.52.

Crystal Structure

Sometimes known as adamant, it is the hardest known naturally occurring material, scoring 10 on the old Mohs scale of mineral hardness. The material boron nitride, when in a form structurally identical to diamond, is nearly as hard as diamond; a currently hypothetical material, beta carbon nitride, may also be as hard or harder in one form. The diamond derives its name from the Greek adamas, "untameable" or "unconquerable", referring to its hardness.

Diamonds typically crystallize in the cubic crystal system and consist of tetrahedrally bonded carbon atoms. A second form called lonsdaleite with hexagonal symmetry is also found. The local environment of each atom is identical in the two structures. Cubic diamonds have a perfect octahedral cleavage, which means that they have four cleavage planes. Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles. Other forms include dodecahedra and cubes. Diamonds are commonly found coated in nyf, a gum-like skin. Their fracture may be step-like, conchoidal (shell-like, similar to glass) or irregular.

Optical properties

The lustre of a diamond is described as adamantine, which simply means diamond-like. Diamonds exhibit fluorescence of various colors under long wave ultra-violet light, but generally bluish-white, yellowish or greenish fluorescence under X-rays. Diamonds have an absorption spectrum consisting of a fine line in the violet at 415.5 nm. Colored stones show additional bands. Brown diamonds show a band in the green at 504 nm, sometimes accompanied by two additional weak bands also in the green. Adamas Gemological Laboratory (http://www.gis.net/~adamas) makes spectrophotometer machines that can distinguish natural, artificial, and color-enhanced diamonds.

Electrical properties

Except for most natural blue diamonds which are semiconductors, diamond is a good electrical insulator, but unlike most insulators, is a good conductor of heat because of the strong bonding within the molecule. Specially purified artificial diamonds have the highest thermal conductivity (20-25 W/cmK, five times more than copper) of any known solid at room temperature. Most natural blue diamonds contain boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductance. Natural blue diamonds recently recovered from the Argyle mine in Australia have been found to owe their color to an overabundance of hydrogen atoms: these diamonds are not semiconductors.

Thermal properties

Because diamonds have such high thermal conductance they are already used in semiconductor manufacture to prevent silicon and other semiconducting materials from overheating. Natural blue diamonds containing boron and synthetic diamonds doped with boron are p-type semiconductors. If an n-type semiconductor can be synthesized, electronic circuits could be manufactured of diamond. Worldwide research is in progress, with occasional successes reported, but nothing definite. In 2002 it was reported in the journal Nature that researchers have succeeded in depositing a thin diamond film on a diamond surface which is a major step towards manufacture of a diamond chip. In 2003 it was reported that NTT developed a diamond semiconductor device.

Composition and color

Type I diamonds have nitrogen atoms as the main impurity. If they are in clusters they do not affect the diamond's color (Type Ia). If dispersed throughout the crystal they give the stone a yellow tint (Type Ib), the Cape series. Typically a natural diamond crystal contains both Type Ia and Type Ib material. Synthetic diamonds which contain nitrogen are Type Ib.

Type II diamonds have no nitrogen impurities. Rarely, they contain no other impurities: these are Type IIa, colored pink, red or brown by structural anomalies arising through plastic deformation. Type IIb are the natural blue diamonds which contain scattered boron within the crystal matrix.

Diamonds occur in a variety of colors - steel, white, blue, yellow, orange, red, green, pink, brown and black. Colored diamonds contain impurities or molecular defects that cause the coloration, whilst pure diamonds are always transparent and colorless.

In the late 18th century, diamonds were demonstrated to be made of carbon by the rather expensive experiment of igniting a diamond (by means of a burning-glass) in an oxygen atmosphere and showing that carbonic acid gas (carbon dioxide) was the product of the combustion. The fact that diamonds are combustible bears further examination because it is related to an interesting fact about diamonds. Diamonds are carbon crystals that form deep within the Earth under high temperatures and extreme pressures. At surface air pressure (one atmosphere), diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (dH = -2 kJ / mole). So, despite De Beers' ad campaign, diamonds are definitely not forever. However, owing to a very large kinetic energy barrier, diamonds are metastable; they will not decay into graphite under normal conditions.