The Right Chemistry: Lab-made diamonds testify to chemists’ ingenuity

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“O Diamond, Diamond, thou little knowest the mischief thou hast done.” Isaac Newton supposedly spoke those words upon seeing his dog Diamond upset a candle and set fire to a manuscript he had been working on for 20 years. While the story of the mischievous dog is likely apocryphal, the fire was real. Newton was very interested in alchemy and had prepared an extensive manuscript on the subject. It was that work that mostly went up in flames, although parts survived. In 2020, three leaves of the scorched document sold at auction for over half a million dollars!

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If Newton did have a dog, he may very well have named it Diamond, however, because the world’s first truly great scientist was interested in diamonds. As Alexander Pope wrote in his famous couplet, “Nature, and Nature’s Laws lay hid in Night, God said, Let Newton be and All was Light.” Cleverly stated, since much of Newton’s work focused on light and how it traveled from one medium to another. His classic experiment showing that white light passing through a prism can be separated into the colours of the rainbow introduced the concept of refraction, the phenomenon of light being deflected as it passes from one medium into another.

In this connection, Newton studied light passing through a diamond and noted that it refracted in a fashion similar to “inflammable bodies” such as olive oil, turpentine, and amber. He conjectured that the diamond is “an unctuous (oily) body coagulated,” thereby becoming the first person to reflect on the chemical makeup of a diamond. Whether he tried to combust a diamond, usually the first step in determining the composition of a substance, is not known, but he very well may have since he was of course interested in lenses and was undoubtedly familiar with their ability to achieve extremely high temperatures by focusing sunlight.

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The brilliant French chemist Antoine Lavoisier did carry out a pioneering experiment along these lines in 1772 when he focused light on a diamond in a closed glass vessel and noted that the diamond was set aflame! He even managed to trap some of the air in which the inflammation had taken place and found that when it was passed through lime water (calcium hydroxide) it formed a precipitate of calcium carbonate. This was a giveaway that the combustion had produced carbon dioxide gas with the carbon coming from the diamond.

Lavoisier was not able to make a quantitative connection between the weight of the diamond and the amount of carbon dioxide produced, but the English chemist Smithson Tennant managed to do exactly that in 1796. He burned a weighed sample of diamond, collected and weighed the calcium carbonate produced and determined that the carbon content of the calcium carbonate was equal to the weight of the diamond that had burned. This meant that the diamond was composed of pure carbon!

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Indeed, today we know that diamond is a crystalline form of carbon in which each carbon atom is bonded to four others in a tetrahedral arrangement, extending in three dimensions. This arrangement makes diamond the hardest known substance. Naturally occurring diamonds were formed billions of years ago deep within the Earth, not from coal as was first surmised, since coal is the end product of decomposing plant or animal matter and diamonds were formed before these ever appeared. The consensus is that diamonds were formed from such minerals as various carbonates upon exposure to high temperatures and pressures in the bowels of the Earth, and were brought to the surface via volcanic eruptions. Diamonds are rare, their mining laborious, polishing and cutting difficult, and demand high, which accounts for their high cost.

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Due to the beauty and scarcity of diamonds, it is understandable that ever since Tennant’s discovery that the gems were made of carbon, scientists have been intrigued by the possibility of producing diamonds in the lab. French chemist Henri Moissan’s interest was triggered by examining rock samples from a meteorite that landed in Arizona that he believed contained tiny diamonds. He later determined that these were actually crystals of silicon carbide, a substance that was almost as hard as diamond. Nevertheless, he believed that the heat and pressure experienced by a meteorite could be reproduced in the lab and possibly produce diamonds.

Moissan developed an electric arc furnace that could reach temperatures in excess of 3500 C in which he placed a crucible made of carbon that contained iron. When molten iron is rapidly cooled, it contracts and produces great pressure, possibly enough to convert the carbon to diamond. Indeed he thought he had successfully produced a diamond but that has been questioned. However, he did go on to make silicon carbide synthetically, and that was almost as good as making a diamond. It gave rise to “moissanite” jewelry, which simulates diamonds. It is far cheaper than diamond and has similar brilliance. Moisson’s other interest was in minerals that contained fluorine, from which he eventually isolated the element fluorine for which he received the Nobel Prize in Chemistry in 1906. Fluorine has numerous uses such as in the production of Teflon and stain and water repellant substances known as perfluoroalkyl substances, PFASs. These are controversial because of their environmental persistence and potential toxicity.

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Real synthetic diamonds were first produced in 1955 by researchers at General Electric who subjected graphite, another crystalline form of carbon, to extreme temperatures and pressures. These are real diamonds, indistinguishable from naturally occurring ones, but cheaper. They should be distinguished from the even more economical diamond imitations like cubic zirconium made of zirconium oxide, or moissanite, which look very much like diamonds but have a different chemical composition.

Many people prefer “real” mined diamonds and look on all others as “fake.” I actually appreciate the “fakes” more, especially synthetic diamonds, given the chemical ingenuity that has gone into making them. There is also no concern about them being “blood diamonds” mined in war zones and sold to finance conflicts.

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Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.