Composition
Diamond is carbon in its most concentrated form. Except for trace impurities like boron and nitrogen, diamond is composed solely of carbon, the chemical element that is fundamental to all life

But diamond is distinctly different from its close cousins the common mineral graphite and lonsdaleite, both of which are also composed of carbon. Why is diamond the hardest surface known while graphite is exceedingly soft? Why is diamond transparent while graphite is opaque and metallic black? What is it that makes diamond so unique?

The key to these questions lie in diamond's particular arrangement of carbon atoms or its crystal structure--the feature that defines any mineral's fundamental properties. A crystal is a solid body formed from the bonding of atomic elements or compounds in a repeating arrangement. Often, crystals possess smooth external faces. Due to their symmetrical and finite nature, the building blocks of crystals are limited to relatively small numbers of atoms, and their chemical compositions to simple numerical combinations of elements.

Chrystal Structure
A neutral carbon atom has 6 protons and 6 electrons surrounding its nucleus. Four of the electrons in a carbon atom are valence electrons, which are electrons that are available to form bonds with other atoms. In graphite, each carbon atom bonds only 3 of its 4 valence electrons with neighboring carbons. The resulting structure of these bonds is a flat sheet of connected carbon atoms. Though individually strong, these layers are only weakly connected to one another, and the ease with which they are separated is what makes graphite so slippery.

This model shows how each carbon atom (ball) is connected to 4 other carbon atoms by strong chemical bonds (rods), creating diamond's rigid crystal structure.

In diamond however, every carbon shares all 4 of its available electrons with adjacent carbon atoms, forming a tetrahedral unit. This shared electron-pair bonding forms the strongest known chemical linkage, the covalent bond, which is responsible for many of diamond's superlative properties. The repeating structural unit of diamond consists of 8 atoms which are fundamentally arranged in a cube.

Real diamond crystals don't have completely smooth faces. Trigons are triangular growths that reflect subtle changes in height on a diamond's face. The trigons shown here are slight indentations that were most likely produced by a natural etching of the crystal. However, raised trigons, which point in the same direction as the crystal face, may also occur from etching, dissolution, or as part of the natural growth of the crystal.

Using this cubic form and its highly symmetrical arrangement of atoms, diamond crystals can develop in a variety of different shapes known as "crystal habits." The octahedron, or eight-sided shape that we associate with diamonds is its most common crystal habit. But diamond crystals can also form cubes, dodecahedra, and even combinations of these shapes. All of these shapes are manifestations of the cubic crystal system to which the mineral diamond belongs. Two exceptions are the flat form called a macle, which is actually a composite crystal, and etched crystals, which have rounded surfaces and, sometimes, elongated shapes.

Hardness
Diamond is renowned for its hardness. Hardness is the measure of a substance's resistance to being scratched, and only a diamond can scratch another diamond. Diamond is the hardest substance known.

The Mohs scale--a hardness scale developed in 1822 by Austrian Friedreich Mohs as a criterion for mineral identification--can help us appreciate the hardness of diamond. The scale ranks 10 minerals; harder minerals, with a higher number, can scratch those with a lower number.

When the mineral hardness numbers from the Mohs scale are plotted against those on the more quantitative Knoop scale (based on the force needed to make indentations using a diamond), we can see how it doesn't adequately express the extreme hardness of diamond. The Mohs scale is relatively stable until it reaches the eighth mineral topaz, but it jumps exponentially from corundum (colorless sapphire) to diamond. It is in fact difficult to measure the hardness of diamond, because diamond must be used to measure its own hardness.

Durability
Hardness is not the only measure of a mineral's durability--the relative resistance to fracture is another. Although diamond is not fragile or prone to breaking apart, all substances including diamond can fracture or shatter. Due to its particular crystal structure, diamond has certain planes of weakness along which it can be split. Diamond is said to have perfect cleavage in four different directions, meaning it will separate neatly along these lines rather than in a jagged or irregular fashion. This is because the diamond crystal has fewer chemical bonds along the plane of its octahedral face than in other directions. Diamond cutters take advantage of cleavage to fashion diamonds efficiently.

Surface Properties
How is diamond like a freshly waxed car? It repels water, an unusual property for a mineral. Diamond's strong bonding and carbon composition cause its surface to repel water but to readily accept wax and grease. These two properties provide an effective means of separating diamonds from other minerals that come out of mining operations. Washed gravel containing diamonds is flushed with water over a sloping surface covered with a mixture of wax and grease, a "grease table." The diamonds stick to the table, while the wetted waste minerals wash over it. Gem diamonds readily pick up a greasy film, but cleaning with ammonia or a good detergent restores their brilliance.

Density
Density is a ratio of a substance's mass to its volume. For instance, density explains why a certain amount of lead feels heavier than an equal volume of salt. Diamond is amazingly dense given the low atomic weight of carbon. At 3.51 grams per cubic centimeter, diamond is much more dense than graphite, which weighs in at only 2.20 grams per cubic centimeter. This comparison offers an important clue to diamond's origin: the fact that diamond's carbon atoms are "squeezed" together tighter than in graphite, which forms near Earth's surface, implies that diamond is formed under high pressure conditions. This concept was corroborated by the experimental synthesis of diamond at high pressure and temperature illustrated on the graph below.

This simplified diagram shows the conditions of pressure and temperature where diamond and graphite will be the stable forms of carbon. The points show the conditions at which diamonds were first grown by the companies ASEA and General Electric in the early 1950s. Temperatures are in Kelvin--subtract 273 to convert to degrees Celsius.

The Eiffel Tower Upside DownThis magnitude of pressure is difficult to comprehend. For example, the pressure of 55,000 atmospheres necessary to make a diamond at 1400 degrees C (orange hot) would require:

* 10 Anighito meteorites (10 X 20 metric tons) resting on a penny.
* The Eiffel Tower (7000 metric tons) resting on a 5 inch plate

Refraction
Diamond's brilliance and luster are two of its most valued attributes. The science behind such phenomena is diamond's great ability to refract light; that is, to bend or slow light as it passes through it. The amount that a substance can impact light in these ways is quantified in its refractive index.

Science postulates the speed of light in a vacuum to be about 186,000 miles per second. But the velocity of light is slowed whenever it is forced to interact with the electrons of a substance, whether it's a liquid, gas or solid. Generally speaking, higher density materials have greater concentrations of electrons and therefore greater capabilities to refract light. Light passing through diamond is reduced to about 77,000 miles per second--near the maximum for any transparent substance.

Reflectance, or the amount of light reflected from a transparent substance, can also be inferred from a material's refractive index. Once again, diamond displays the maxim amount of reflectance for a transparent substance, displaying what is called an "adamantine" luster.

Color
Our standard conception of diamond is as a colorless stone. But color in diamond exists in myriad variations, from dazzling pinks and yellows to blues and violet. A chemically-pure, perfect crystal of diamond is colorless, but add a little nitrogen and yellow appears. Add boron instead and a blue diamond results. Colors from red to violet, real white, and black are possible and can be complex to understand scientifically. Colored diamonds are hot, both in the marketplace and in science.

To understand color in diamond, one must remember that light is a form of energy. In this representation of the visible spectrum, low energy is at the bottom and high energy at the top. Each color of the rainbow corresponds to a particular energy. When the energy of light entering a diamond equals the amount needed to bump an electron to another configuration, parts of the spectrum are absorbed. A pure diamond is colorless because visible light lacks sufficient energy to excite any of its electrons and therefore no light is absorbed. However, impurities like nitrogen, boron or hydrogen, as well as structural flaws, can create electron states which can be effected by the energy in visible light. In the diamond pictured above, the higher energies of violet and blue light are subtracted from white light making the diamond appear yellow.

Dispersion
The glinting spectrum or "fire" from a colorless diamond--one of its most prized attributes as a gemstone--results from its excellent dispersion. Dispersion is the separation of white light into its component rainbow colors. The greater the dispersion, the greater the separation between the spectrum of colors that are refracted from a gem.
Dispersion

The refractive index can also be used to describe how visible light can be split into the colors of the spectrum when passing through a transparent substance. Essentially, this happens because the refractive index of a substance is not constant, but rather varies for different wavelengths, or colors, of light. Consequently, the shorter wavelengths of light (the blue end of the spectrum) are bent more than the longer wavelengths (the red) when entering a colorless substance at an angle. Thus, the colors separate, or disperse, producing the visible spectrum as from a prism. The coefficient of dispersion is a measure of this variation. The greater a substance's coefficient of dispersion, the greater the angular spread of colors from an incoming, inclined beam of white light--a characteristic described as a gem's "brilliance."

Fluorescence and Phosporescence
An interesting property of some diamonds is that they can glow in the dark. When illuminated by ultraviolet light, certain diamonds can absorb the high-energy radiation and re-emit it as visible light. These diamonds are called fluorescent. Some can even continue glowing after the ultraviolet source is turned off. These diamonds are phosphorescent.

This 2.15-carat, cushion-cut diamond fluoresces in daylight but not in incandescent light, such as that given off by a light bulb. It is a color-change diamond, going from intense, greenish yellow in fluorescent light or daylight to yellowish brown in incandescent light. This Brazilian diamond has a historic pedigree, having been given by Pedro II de Alcantara (1825-91), emperor of Brazil, to his niece. Dom Pedro II, a descendant of the Hapsburg family and the Braganza royal family that ruled Portugal, ascended to the throne of Brazil in 1831 and ruled until 1889. He was a scientist in his own right and was greatly interested in diamonds, mineral specimens, and geology.

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