SciTech

How Things Work: Diamonds

Millions of years ago, diamonds were formed about 161 kilometers below the earth’s surface in a section known as the mantle. Only in the mantle are conditions intense enough to pressurize pure carbon into a gem. Diamonds are made when the pressure reaches 435,000 pounds per square inch and the temperature is above 752°F. Anything below these figures results in graphite, a softer, flakier carbon composition.

The diamonds that we find today reached the Earth’s surface by powerful magma eruptions through deep cracks. The eruptions formed pipes called Kimberlite pipes, after the first pipes found in Kimberley, South Africa. These are also known as diamond pipes. Diamond miners look for kimberlite, a bluish rock that resulted from the cooled magma, when looking for diamonds. However, diamonds can be found in other locations due to geological activity and glacier and water movement.

Though diamonds are valued for their beauty and rarity, they are, in fact, no rarer than any other gem or precious stone; diamonds are simply carbon. They are special because they are composed of carbon in its most concentrated form, making it the hardest naturally found substance on earth.

Diamonds are hard because of their unique molecular structure. The carbon atoms in a diamond are closely packed in a crystal lattice arrangement of tetrahedrons, in which a central carbon atom is covalently linked to four surrounding carbon atoms. Though the thought of four linked objects calls to mind a square, a tetrahedron is actually triangular. This shape takes less energy to maintain than a square and is also sturdier. If you try to tie four balloons together, they do not naturally fit as a planar square — the balloons will rearrange themselves into a triangular shape.

Diamonds are sturdy because of their atomic arrangement, but why are they also so hard and resilient? The answer lies in one of the smallest subatomic particles, electrons. Carbon atoms are surrounded by a total of six electrons. These electrons are arranged in three orbitals, the innermost orbital having two electrons, and the two outermost orbitals having four, known as valence electrons. Carbon is short two electrons, which would fill the outer orbitals with unpaired electrons and create a stable octet. Eight electrons is the maximum number of electrons that carbon’s valence shell can hold.

When carbon atoms come together in a diamond, they arrange themselves into a lattice by forming covalent bonds in which they share unpaired electrons with adjacent atoms. Electrons that were whizzing around a single atom now travel and disperse their energy between two atoms. Each carbon atom in a diamond is, in effect, sharing its four valence electrons in order to create a stable octet. The structure that results is very stable, giving diamonds their characteristic hardness.

Truly rare diamonds are those of pure carbon and are colorless. Other rare diamonds are colored and can be yellow, red, orange, blue, green, brown, or black. Besides these, the “rarity” of diamonds has been engineered by diamond cartels that monopolize the mines and the market. For the past several decades, De Beers Consolidated Mines, Ltd., based in Johannesburg, South Africa and London, has maintained a firm grip on the diamond market.

Diamonds were not always so popular. Emeralds, rubies, opals, and pearls, among other gems, were equally valued and were used often as engagement or wedding rings because of their rich colors and exotic appearances. The diamond market did not fare well during the Great Depression, particularly since antitrust laws in the U.S. forbidding cartels prevented De Beers from directly selling diamonds. However, a brilliant marketing campaign launched in 1938 by N. W. Ayer & Son re-imagined the diamond as a symbol of eternity and love with their slogan “A diamond is forever.”

The campaign accomplished two things. First, it cast diamonds as the only jewels for special occasions, particularly engagements, weddings, and anniversaries. Second, it encouraged women to keep their diamonds. This prevented competition from a secondary market of used diamonds and allowed retailers to keep prices high. Indeed, the markup on a mounted diamond is between 100 and 200 percent.

Though there may be a diamond monopoly on earth, there are also diamonds in the sky. Tiny but numerous diamonds have been found embedded in meteorites. Often the chemical makeup is the same. However, some of these extraterrestrial variants include xenon gas; this combination is not found on earth. Still others show a hexagonal atomic structure that is unlike the cubic structure of diamonds on earth. Scientists posit that these diamonds were formed during intense extraterrestrial impacts.

Next time you see a diamond, think about all the hard work and time that was put into making it!