The iconic advertising slogan “A Diamond is Forever” has been tied to the diamond industry since 1948. Over the years it’s inspired similarly titled books—and even a James Bond film. But is it really true? More broadly, what does science have to say about the gemstone that became the ultimate symbol of love?  Let’s take an in-depth look at the properties of diamonds—and, along the way, find out who might not necessarily be thrilled to receive one as a gift.


What does science have to say about the gemstone that became the ultimate symbol of love? Diamond ring | Shutterstock, Aleona

A Single Element, a Remarkable Structure

Let’s start with a bit of chemistry: all matter in the universe is made of tiny particles called atoms. Diamonds are no exception—they’re built entirely from carbon atoms. The force that holds atoms together in larger structures is called a chemical bond. There are several types of chemical bonds, and the one that matters here is the covalent bond: inside every atom are subatomic particles called electrons, and when two atoms share electrons, they form a covalent bond.

Carbon is distinctive when it comes to covalent bonding. Different elements can form different numbers of such bonds, and carbon can form four—more than most.

Two or more atoms connected by covalent bonds are called a molecule, and the molecule of a diamond is especially remarkable. Because a diamond is made only of carbon atoms, each carbon atom is bonded to four other carbon atoms. Each of those is bonded to four more in every direction, which are bonded to four additional carbon atoms, and so on—forming a structure known as an atomic lattice. This structure allows more and more carbon atoms to join the lattice, until an enormous molecule forms—one you can actually see with the naked eye. By comparison, most molecules are so tiny that you can’t see them even with a microscope. A water molecule, for example, is about 0.3 millionths of a millimeter across. So when you look at a diamond, you are, in effect, looking at a single, complete molecule.

 A diamond contains countless carbon atoms bonded together in a three-dimensional structure. The spheres represent atoms and the lines represent chemical bonds | Shutterstock, Andris Torms, Vertyr

 

The Hardest in Nature

Carbon appears in nature in many forms—for example graphite, which is also made entirely of carbon atoms. Yet the two materials couldn’t be more different: diamond is the hardest natural substance, while graphite is very soft. When we write with a pencil, we’re actually smearing soft graphite onto the paper. The difference comes down to how the carbon atoms are bonded. In graphite, each carbon atom is symmetrically bonded to only three neighboring carbon atoms, forming flat, connected hexagons—like a honeycomb.

The atomic structure of carbon in graphite—honeycomb-like hexagons stacked one on top of another | Shutterstock, Peter Hermes Furian

But if carbon can form four bonds, what happens to the “extra” bond in graphite? Chemical bonds involve interactions between atoms’ electrons. In graphite, one electron per carbon atom isn’t locked into a single bond; instead, these electrons are shared across the structure and can move through it. Because mobile electrons carry electric current, graphite is able to conduct electricity:

Within each graphite layer, carbon atoms are held together by strong covalent bonds. Between the stacked hexagonal sheets, however, there are no covalent bonds—only much weaker attractions—so the layers can slide over one another with very little force. That’s why graphite flakes off and smears onto paper.  In diamond, by contrast, every carbon atom is strongly bonded to others in all directions, creating a rigid 3D network where the atoms are locked in place in all directions.  Silicon can also form four covalent bonds and produces hard solids, such as quartz (silicon dioxide), but none are as hard as diamond.

Graphite’s layers slide past one another with very little force, which is why it smears onto paper. Graphite | Dirk Wiersma / Science Photo Library

We’ve said diamond is hard—but what does “hard” actually mean? Materials have several kinds of “strength,” and everyday language doesn’t always distinguish between them.  In geology, hardness usually means resistance to scratching—the property measured by the Mohs scale. By that definition, diamond is exceptional: almost nothing can scratch it, except another diamond. The Mohs scale is essentially a “scratch contest,” comparing pairs of materials to see which one scratches the other—and diamond comes out on top. This extreme hardness also gives diamonds important industrial uses: saw blades and drill bits designed for cutting very hard materials sometimes contain diamond grit, which can scratch and grind the target material away.

But hard doesn’t mean tough. A diamond can still break if it takes a strong blow—something diamond cutters (unfortunately) know all too well. In fact, very hard materials are often brittle. Glass, for example, is hard enough that it’s commonly cut with a diamond, yet it shatters easily. Even a hard steel knife can snap if you bend it far enough. At the other end of the scale, dough and modeling clay are so soft that they deform rather than fracture when dropped or twisted. So despite its hardness, a diamond can crack, break, and even shatter into fragments. In that sense (and in other ways we’ll see later), it doesn’t last forever.

Saw blades and drill bits used to cut and drill extremely hard materials often contain diamond grit. A diamond-coated blade cutting granite | Science Stock Photography / Science Photo Library

From Total Internal Reflection to Sparkle

When you look at diamond jewelry, you almost always see sparkles of light from within. That sparkle comes from the diamond’s optical properties, enhanced by the way the stone is cut and polished. Up close, a diamond is transparent—light can pass through it—but that’s only part of the story: some of the light that enters the diamond is reflected and redirected back out, often toward the viewer, rather than continuing straight through.

When light passes from one material to another—say, from air into glass or from air into diamond—its speed changes. That change makes the ray bend at the boundary, a phenomenon called refraction. The degree to which light slows down in a material is described by its refractive index, and diamonds, thanks to their tightly packed atomic structure, have one of the highest refractive indices in nature. One consequence is total internal reflection: light can enter the diamond, but when it later strikes an internal surface at a shallow enough angle, it cannot escape into the air. Instead, it is reflected back inside the stone, remaining trapped within it.

Diamond has an especially high refractive index, which means its critical angle for total internal reflection is small – about 25°. As a result, light traveling inside the stone that reaches a facet at a shallow enough angle is reflected back inward rather than escaping. Diamond cutters take advantage of this by shaping diamonds with many facets at carefully chosen angles, so that in almost any position at least one facet directs the reflected light back toward the viewer—making the diamond appear to sparkle.

In a well-cut diamond, at least one facet is almost always reflecting light back toward the viewer, creating sparkle.| Shutterstock, Rashevskyi Viacheslav

Forever?

Diamonds are made, as we’ve seen, of carbon—an element that can burn. Barbecued charcoal, for example, is also made of carbon. Does that mean diamonds are combustible too? It may sound surprising, but the answer is yes: when diamonds are heated and exposed to pure oxygen, they will burn.

Nearly three centuries ago, the renowned chemist Antoine Lavoisier, considered one of the founders of modern chemistry – demonstrated this experimentally. He identified the gas released in the process as carbon dioxide, thereby proving that diamond itself is composed of carbon.

So in this sense, diamonds are not forever—they can burn—something you wouldn’t expect from a gemstone

But diamonds are not “forever” in another, deeper sense as well. If we return to graphite and compare its stability with that of diamond, we find something intriguing. To examine the stability of materials, we can turn to thermodynamics, the branch of physics that studies energy and change. One of its guiding principles is that systems tend toward a thermodynamic minimum—a more stable state associated with lower energy (and, loosely speaking, greater disorder). In thermodynamic terms, graphite is more stable than diamond. Because natural systems tend to move toward more stable states, every diamond will eventually transform into graphite. As one lecturer I studied with liked to say, a thermodynamicist will never buy his wife diamonds.

Thermodynamics can tell us what direction change will take, but not how fast it will occur. Diamonds may indeed turn into graphite one day—but on timescales of billions of years. In practice, there’s no need to worry that a diamond ring will turn overnight into a ring set with pencil tips.

No need to worry that a diamond ring will turn overnight into a pencil ring. A diamond dealer examines a polished diamond | Shutterstock, EgolenaHK

How Is a Diamond Born?

In nature, diamonds form deep within the Earth under conditions of immense pressure and heat. At depths where pressures can reach about 60,000 atmospheres—roughly 60,000 times the air pressure at Earth’s surface—and temperatures rise to around 1,500 °C, carbon-rich materials such as graphite can transform into diamond. These extreme conditions exist deep in the Earth’s mantle, and the process typically unfolds over millions to billions of years. 

Occasionally, rapid volcanic eruptions carry diamonds formed at depth toward the surface, sometimes bringing them close enough to be mined. Because of this long and complex journey, diamonds are relatively rare and their extraction can be challenging. In the past, diamond mining has been linked to violent conflict and human suffering, particularly in parts of Africa—a reality portrayed in the film Blood Diamond.

In recent decades, scientists and commercial companies have sought to produce diamonds synthetically, both to reduce costs and to avoid human rights concerns. Early efforts attempted to replicate the natural process by subjecting graphite to extremely high temperatures and pressures. Although this method successfully produced diamonds, they were not cheaper than natural stones and could not match the most valued gems—the large, clear diamonds prized in jewelry.

Over time, magma rising from deep within the Earth can carry diamonds formed at great depth toward the surface. Rough (uncut) diamonds | Shutterstock, EgolenaHK

From the Ground to the Lab

The breakthrough that has become commercially viable over the past two decades uses a completely different approach—one that bears little resemblance to natural diamond formation. This method is called CVD, short for Chemical Vapor Deposition. Instead of transforming an existing carbon material, the goal is to build a diamond from its most basic building blocks: carbon atoms.

The principle is straightforward. A carbon-containing gas (typically methane) is passed through a chamber and heated to a high temperature in the absence of oxygen, so it doesn’t burn. At sufficiently high temperatures, the gas molecules break apart into their constituent atoms—hydrogen and carbon. A small diamond “seed” is placed at the end of the chamber, and the carbon atoms in the gas settle onto its surface and attach to it, atom by atom. In effect, the diamond grows layer by layer, a process similar to crystal growth

Building a diamond from its most basic components—carbon atoms. A synthetic diamond produced using chemical vapor deposition | Wikipedia, Steve Jurvetson

By fine-tuning the growth conditions, manufacturers in the diamond industry can now produce large diamonds without extreme pressures and without harming anyone. The relative simplicity of the process has led to a dramatic drop in the price of synthetic diamonds in recent years. Importantly, a synthetic diamond is not a fake—it is a diamond in every physical and chemical sense, with the same remarkable properties and without the human cost associated with some mining practices. Even a professional jeweler cannot distinguish a lab-grown diamond from one formed underground by sight alone; only specialized instruments that analyze the isotopic composition of the carbon atoms can determine a diamond’s origin with certainty.

To Buy or Not to Buy?

We’re a science website, so we can’t answer that question—it goes far beyond science alone. The diamond industry is vast, and in addition to its scientific aspects, it involves social, economic, marketing, psychological, and ethical considerations. All of these may factor into a decision about whether – and which – diamond to buy.