diamonds -- 8/6/26

06 Aug 2026

diamonds -- 8/6/26

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Today's encore selection--from Precious by Helen Molesworth. The formation of diamonds.

“How diamonds like the Cullinan came close enough to the earth's surface for humans to discover and extract is the product of two geological marvels. The first is the formation of the crystal itself, the compression of carbon atoms under conditions of sufficient heat (approximately 1,200°C) and pressure (more than 40,000 atmospheres) that they combine into structures of immense strength—able to survive almost anything that nature can throw at them. The diamond's innate hardness is deep-rooted: whereas almost all other gemstones form within the earth's crust, the diamond's origin is way down in the mantle, several hundred miles below the surface.

“The formation of the diamond is just the beginning of its high octane journey. The next leg follows courtesy of volcanic eruptions that begin deep underground, where the melting of the mantle creates a magma rich in carbon dioxide, one that explodes upward like the contents of a shaken bottle of fizz. This underground jet gains impetus as it goes, with xenoliths (literally ‘foreign rocks’) including silicates and diamonds joining the mix. These mineral additions are the geological equivalent of the effect of dropping Mentos into a bottle of Diet Coke—causing the release of carbon dioxide, making the magmatic mixture more acidic and ultimately helping it foam and explode upward, toward and through the surface. The magnitude of such volcanic events can only be estimated, since most eruptions of this kind happened between 70 and 150 million years ago. The diamonds they carried were even older: between 1 and 3 billion years old. Within the context of the history of the earth itself—approximately 4.6 billion years old—diamonds are ancient.

“The result, once cooled, is a carrot-shaped intrusion of a rock known as kimberlite, after Kimberley in South Africa's Northern Cape, where primary diamond deposits of this kind were first discovered in 1871. These kimberlite ‘pipes’ are the natural world's delivery system for diamonds. Only a small percentage of pipes will contain any meaningful quantity of diamonds and only a fraction of those are commercially exploitable. The modern method of extracting them entails the heavy industrial blasting that I observed at the Cullinan Mine, followed by the crushing and processing of kimberlite ore, breaking it down into smaller and smaller pieces so as to discover whether diamonds are hiding within. Yet for most of the diamond's history its geological origin was unknown and discoveries were of stones that had been eroded and washed away, mined alluvially from extant or long-buried riverbeds. Some of those diamonds were washed into the ocean, where they are recovered today by specially designed ‘crawler’ ships that dredge and sift gravel from the seabed.

“Diamonds are formed under conditions of extreme heat and pressure, and they are brought into our realm by volcanic eruptions without parallel in the earth's modern history. It should be no surprise that the resulting stone—the one that has survived all this—is unique, with physical and chemical qualities that explain its place at the peak of the precious gem hierarchy.

A natural diamond crystal

“It is its hardness for which diamond is most famous. Sitting at the top of the Mohs scale (which ranks how comparatively resistant a mineral is to scratching), diamond scores a perfect 10, above ruby and sapphire at 9.

“Yet that understates the level of difference: in absolute terms, the diamond can be more than four times as hard as corundum, courtesy of its tight and symmetrical atomic structure. This hardness—only a diamond can scratch another diamond—explains one secret of the diamond's beauty. It allows it to be polished to a level unlike any other, giving it supreme and adamantine luster (the reflection of light off the surface). The polishing—a separate process to the cutting, performed by its own specialists— can be a major undertaking in its own right: for Cullinan I, it began in March 1908 and was not finished until September, with a team of expert polishers working fourteen-hour days to complete the job.

“Yet hardness is only part of the diamond's secret formula. What makes the stone truly visually remarkable is its high refractive index, the ability to slow down and bend the light waves entering it. If the gem is cut correctly, light entering it will bounce off multiple internal surfaces, break up into its component color waves (the dispersion of light known as a diamond's ‘fire’), and bounce back out into the eye of the onlooker, creating an internal reflection we call brilliance. This combination of reflection, refraction, and dispersion is at play in all gemstones, but never better than with a diamond that has been cut and polished to perfection. There is no secret to the diamond's longstanding reputation as the King of Gems: the chemical and physical marvel of its creation gives it an appearance and a relationship to light that is both magical and unmatched.

“It is the simplicity of the diamond that makes it so stunning—the perfection that arises from carbon alone, the atoms covalently bound together in a cubic structure, with perfect symmetry along all planes and axes. Yet carbon is not quite on its own within the structure of a diamond. Almost all natural diamonds— 95 percent—contain traces of nitrogen in small clusters (Type Ia), which can lend the diamond a faint ‘cape’ yellow tinge. A much smaller subset (Type Ib) have isolated nitrogen atoms that imbue the stones with a bright ‘canary’ yellow hue.

“It is the minority without any impurities at all (Type IIa) that are the truly special diamonds, which make up almost all the world's most famous and highly valued examples, including the Cullinan. Like the Cullinan, Type Ila diamonds are often completely colorless, the whitest of the white, with wonderful transparency. These are remarkable gems, with a purity and crystalline clarity that make you feel as though you are looking into a perfect, limpid pool of water. Even more exceptional is the small percentage of these Type Ha diamonds with structural defects in the crystal lattice that create a pink color. The final category (Type IIb) contain traces of boron, which can turn the diamond blue and allow it to conduct electricity.”