Diamonds are carbon in its most concentrated form. Except for trace impurities like boron and nitrogen, diamonds are composed solely of carbon, the chemical element that is fundamental to all life.
But diamonds are distinctly different from their close cousins, the common mineral graphite and lonsdaleite, both of which are also composed of carbon. Why are diamonds the hardest surfaces known, while graphite is exceedingly soft? Why are diamonds transparent while graphite is opaque and metallic black? What is it that makes diamonds so unique?
The key to these questions lie in a diamonds 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.
Hardness & Durability
Diamonds are renowned for their hardness. Hardness is the measure of a substances resistance to being scratched, and only a diamond can scratch another diamond. Diamonds are the hardest substances known.
Hardness is not the only measure of a mineral's durability--the relative resistance to fracture is another. Although diamonds are not fragile or prone to breaking apart, all substances including diamonds can fracture or shatter. Due to their particular crystal structure, diamonds have certain planes of weakness along which they can be split.
Brilliance & Luster
Diamonds brilliance and luster are two of its most valued attributes. The science behind such phenomena is a diamonds 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 diamonds is reduced to about 77,000 miles per second--near the maximum for any transparent substance.
Our standard conception of diamonds is as a colorless stone. But color in diamonds exists in many 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 blue diamonds result. 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.
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.
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.
Diamonds are called "ice" with good reason. Objects feel cold not only because they are at a lower temperature than our bodies, but also because they can extract or conduct the heat away from us. When you touch a diamond to your lips, it feels ice-cold because it robs your lips of their heat. The capacity of diamonds to conduct heat distinguishes them readily from other gems and exceeds that of copper, an excellent thermal conductor, by about 4 times at room temperature. This exceptional property of diamonds is increasingly being used for extracting heat from electronic devices to make them smaller and more powerful.