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Diamonds typically crystallize in the face-centered cubic crystal system (space group ) and consist of tetrahedrally bonded carbon atoms. The unit cell of diamond has a two atom basis at (0,0,0) and (1/4,1/4,1/4), which means half of the atoms are at lattice points and the other half are offset by (1/4,1/4,1/4), where 1 is the length of a side of the unit cell. Diamond’s density is 3.52 g·cm−3.

The tetrahedral arrangement of atoms is the source of many of diamond’s properties. The carbon atoms in Graphite, the other major allotrope of carbon, display a different (nontetrahedral) connectivity and as a result shows dramatically different physical characteristics: graphite is a soft, dark gray, opaque mineral. Other elements of the carbon group such as silicon crystalize like diamond.

Lonsdaleite is a polymorph of diamond (and a distinct mineral species) that crystallizes with hexagonal symmetry. It is rarely found in nature but is characteristic of synthetic diamonds. A cryptocrystalline variety of diamond is called carbonado. A colorless, grey or black diamond with a tiny radial structure is a spherulite.

Hardness
Diamond is the hardest natural material known, 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. However, aggregated diamond nanorods, an allotrope of carbon first synthesized in 2005, are now believed to be even harder than diamond.[1]

The hardest diamonds in the world are from the New England area in New South Wales, Australia. These diamonds are generally small, perfect to semiperfect octahedra, and are used to polish other diamonds. Their hardness is considered to be a product of the crystal growth form, which is single stage growth crystal. Most other diamonds show more evidence of multiple growth stages, which produce inclusions, flaws, and defect planes in the crystal lattice all of which affect their hardness (Taylor et al. 1990).

Industrial use of diamonds has historically been associated with their hardness; this property makes diamond the ideal material for cutting and grinding tools. It is one of the most known and most useful of more than 3,000 known minerals. As the hardest known naturally occurring material, diamond can be used to polish, cut, or wear away any material, including other diamonds. Common industrial adaptations of this ability include diamond-tipped drill bits and saws, or use of diamond powder as an abrasive. Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most other diamonds, which are excellent electrical insulators. Industrial-grade diamonds are either unsuitable for use as gems or synthetically produced, which lowers their price and makes their use economically feasible. Industrial applications, especially as drill bits and engraving tools, also date to ancient times.

The hardness of diamonds also 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 every day.

Toughness
Unlike hardness, which only denotes resistance to scratching, diamond’s toughness is good in clevage directions, exceptional in all other directions. 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. Diamond is therefore more fragile in some orientations than others.

Color
Diamonds can occur in nearly any color, though yellow and brown are by far the most common. “Black” diamonds are not truly black, but rather contain numerous dark inclusions that give the gem its dark appearance. Diamonds with a detectable hue other than yellow or brown are known as colored diamonds. If the color is strong enough, a stone may be referred to as a fancy colored diamond by the trade. 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 slight to intense yellow coloration depending upon the type and concentration of nitrogen present. The color scale for colorless diamonds goes from ‘D’ (colorless) to ‘Z’ (dark yellow).

Thermodynamic stability
Like other forms of carbon like coal, diamonds will burn at approximately 800 degrees Celsius, providing that enough oxygen is available. The combustability of diamond was shown in the late 18th century[2] and previously described during Roman times. The diamond phase of carbon is metastable with respect to the graphitic phase under normal conditions; that is, graphite is thermodynamically favored over diamond (ΔG = −2.99 kJ / mol). However, the rate of conversion from diamond to graphite is extremely slow due to the presence of a large kinetic barrier to this rearrangement. At room temperature, it would take an extremely long time (possibly more than the age of the Universe) for an appreciable amount of diamond to decay into graphite.

Tags: Diamonds, Materials

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