by Joel E Arem, PhD, FGA
Everybody loves diamonds. We love them for their hardness, their brilliancy, and their amazing and unique luster. Most people think of diamond as a 'white' (colorless) gemstone, and indeed most gem diamonds are white, or nearly so. But the most valuable of all diamonds are the ones that display a strong color, such as yellow, pink or blue. Many other gems, including tourmaline, beryl and corundum, are also colorless when 'pure' but can display a huge variety of colors, all of which are ‘color varieties’ with distinctive gemstone names.
Why is this so? How can a colorless stone be turned into a vividly saturated and colorful material, so beautiful that it is prized as a gemstone? In order to understand this transformation you need to get small. REALLY small. You have to think of yourself as an atom inside of a growing crystal.
We think of atoms as the tiny ‘building blocks’ of everything we see in the universe. There are more than 100 different kinds of atoms (only 92 occur naturally) and they are all made up of the same components: protons (particles that have a positive electric charge), neutrons (that have no charge) and electrons (which have a negative electric charge). The protons and neutrons ‘live’ in the atom’s nucleus, a microscopic dot in the center.
The ‘standard’ textbook image of the electrons pictures them as a bunch of even tinier dots that spin around the nucleus at specific distances. In this image, the atom is mostly empty space. In fact, it has been suggested that if an atom were the size of a football stadium, the nucleus would be the size of a marble sitting on the 50-yard line.
But this “spinning electron” picture is wrong. And it doesn’t let you understand the cause of color in gemstones.
You have to imagine the atom as something like a cotton ball. The cotton that is close to the tiny nucleus is just a little bit compacted, but moving outward what you see is a ‘fuzz’, a ball of wispy ‘fluff’ that represents the positions of all the electrons. Scientists believe that the negative electric charge of the electrons surrounds the nucleus like a cloud, not tiny particles that spin around it. Some of the electrons prefer to be at specific distances from the nucleus, and so the cloud will have some thin zones that are slightly denser, representing a higher probability of finding an electron in those layers. It’s as if the electrons were everywhere at the same time. And the position of any one electron in the ‘cloud’ is only measurable in terms of the chance of finding the little beast in any one particular place!
When a diamond crystal forms, atoms of carbon come together and assemble themselves in a pattern, a three-dimensional construction that some people liken to a playground ‘jungle gym’. It’s main feature is the presence of a structural unit with a specific shape and size, repeated endlessly in all three dimensions. Think of building a house entirely with bricks that are all exactly the same size and shape and you get the basic idea.
The carbon atoms are like little cotton balls. When they clump together to make a crystal of diamond, the little balls squeeze together. The outer layers of the cotton merge together, and the crystal starts to look like a single homogeneous fluff-ball. The unit of structure that repeats in all directions is determined by the positions of the tiny nuclei at the centers of the balls. Some people draw pictures of crystals with the atoms represented as little spheres, and the structural unit is represented with little sticks connecting the balls to create a “unit” with a specific size and shape. But if you do that, you don’t get the fun of visualizing what really happens when light goes through the crystal.
One of the defining characteristics of all crystals is symmetry. Every crystal has multiple symmetry features that make every solid material unique. The ‘unit cell’ that creates the endless periodicity in three dimensions has a geometric shape with symmetry elements that have been understood and described for centuries. At each point in the unit cell is generally an atom; but in many crystals there is a cluster of atoms, and this little grouping can have its own kind of symmetry elements. The combination of these two features allows for quite a variety of materials!
One requirement of all crystals is that the unit cells pack together to fill space with no gaps. There are only a handful of shapes that allow this to happen. These have been classified, based on the relative lengths and the intersecting angles of the edges of the basic defining unit shape, as crystal systems. Within the systems are additional symmetry elements that subdivide them into crystal classes. All known minerals (and solids in general) are described in terms of their symmetry and the atoms that determine their chemical makeup, and no two combinations are exactly alike. The unique attributes of chemistry and structure are what define any material we call a solid.
In the case of a diamond, the symmetry is that of the so-called ‘isometric’ or ‘cubic’ crystal system. An essential feature of this system is extremely high symmetry, which means that the structure looks the same when viewed from many different angles. High symmetry also means that the fuzzy electron clouds around all the atoms also have similar properties in all directions. This produces a very interesting result that becomes apparent when we consider light passing through a crystal.
Nobody knows exactly what energy IS, although we can describe its properties and effects. The universe began (as far as we can tell) as an incomprehensibly hot expanding ball of energy (the ‘Big Bang’). After only one second the temperature had dropped sufficiently for the energy to start condensing and transforming into a different manifestation, one we call matter. So the universe basically consists only of energy, of various kinds, and with various properties. Einstein proved that energy and matter are, in fact, interchangeable. The same ‘stuff’, in different forms.
Scientists have conveniently characterized the energy that travels through space in terms of a ‘spectrum’ of levels, and the energy itself can be described in terms of either particles or waves, depending on how it is observed and measured. A very, very tiny portion of the energy spectrum (usually called the ‘electromagnetic spectrum’ because it has properties that can be both electrical and magnetic) can be sensed by living creatures. Humans ‘see’ an even smaller part of it called ‘visible light’ (we ‘feel’ another part of it as heat (infrared), and our skin reacts to yet a different part (ultraviolet)..…..by getting sunburned!). Still other parts of the spectrum that have different levels of energy are called radio waves, gamma rays, X-rays, etc. Each level of energy in the entire spectrum can be characterized by a measurement of its ‘wave length’ – the distance between peaks (and troughs) in the wave as it passes by. Think of the waves moving toward an ocean beach, and the space between the crests defines the length of the wave. Long wavelengths carry less energy than short ones. This makes sense because a shorter wave length means that more waves (more energy) can pass a given point in any unit of time.
Light moves through the universe very fast. In fact, the speed of light is the quickest way to get from point A to point B that we know of. Light moves the fastest in a vacuum, slightly slower in air. But when a beam of light enters a solid material it has a problem. The energy contained in the light interacts with the energy of the electron cloud that fills the space between the atomic nuclei, and the beam slows down. It also changes direction, a phenomenon we call refraction.
The light we call ‘white’ is actually a mixture of ALL the wavelengths between infrared (heat) and ultraviolet (which causes sunburn), and these wavelengths are perceived by us as colors. Water droplets in the air can separate these because each color (wavelength) is refracted (bent) a different amount, and the result is a rainbow.
The color we see in ANYTHING is a result of how white light is affected by the material it passes through. In the case of a solid, like a crystal, the interaction is complex and produces some very interesting and wonderful effects.
The high symmetry of a cubic crystal causes the electrons in the structure to settle into configurations that are essentially uniform in all directions. It’s as if the electron ‘fuzz’ acted like a sponge, where a path in any direction was accommodated by a network of open pores. This becomes important if we examine white light and describe it in a particular way.
Imagine a beam of white light as a stream of tiny ‘jimmies’ (growing up in Brooklyn we called them ‘sprinkles’), the multicolored chocolate bits in which you dip an ice cream cone. Every possible color is represented in the mix, but they come in different sizes and shapes. The red ones are long and skinny, and the purple ones are short and stubby. All the other colors have shapes and sizes in between, with the orange ones being slightly shorter and fatter than the red ones, etc. Now imagine that the stream of colored jimmies enters a ‘cotton-ball’ crystal and tries to work its way through the electron ‘fuzz’.
If the crystal is ‘pure’ and made up only of carbon atoms, all settled nicely into their appropriate positions in the structure, the colored jimmies can move through the ‘pores’ in the electron ‘sponge’ unimpeded, in all directions, and all the colors emerge as they were in the original light beam. The crystal looks colorless, or ‘white’. But what happens if there is a disruption in the neat and tidy symmetry of the structure?
All mineral crystals form in places where there are disturbances, both chemical and physical. Geological environments are filled with atoms of various kinds, as well as movements due to changes in temperature and pressure, and also geological disruptions. When a crystal grows there is a kind of microscopic stampede. The atoms that properly make up the material race to find their appropriate positions in the structure, bumping other atoms out of the way and trying to prevent them from contaminating its pristine purity. In some cases the growth of the crystal is so rapid that a structural position may remain unoccupied by any atom. Layers of newly depositing material quickly enclose the defect in the structure, creating what is called a ‘vacancy’. In other cases, an atom of an element that is not normally part of the structure, called an ‘impurity’, is trapped in the growing crystal. The impurity atom might be larger or smaller than the kind that should normally be in that structural position. The difference in size causes the structure to ‘flex’, either outward (impurity is too big) or inward (impurity atom is smaller than the kind that should be in that structural position).
Sometimes clusters of impurity atoms, like gang members, crowd their way in and bully themselves onto a place in the lattice that should be occupied by a single atom. Sometimes, as in the game of ‘musical chairs’, an impurity atom just has nowhere to go as the crystal rapidly forms around it, and the atom ends up squeezing into an entirely inappropriate place between atoms in the structure. We call this an ‘interstitial”.
Sometimes a growing crystal is subjected to squeezing forces that try to bend it out of shape. The pressure creates so much stress that planes of atoms in the crystal may slip and slide over each other, forming a layered mess that is kind of like the irregular pile of offset blocks you get when a row of carefully positioned upright dominoes is pushed over. The process is known as plastic deformation.
ANY of these disturbances can have a profound effect on the ‘fuzz’ of electrons in the structure. ANY of them can disrupt and block the pathways through the spongy matrix of electrons, routes that are normally open to the unimpeded flow of the myriad colored ‘jimmies’ in the beam of white light. Some of them get stuck on their way through, and are taken out of the light beam. The light is no longer ‘white’ because it is no longer a uniform mix of all the wavelengths that our senses are capable of seeing. Instead, the emerging light has a color. The color is ‘white’ MINUS all the wavelengths that were trapped inside the crystal by the distortions in the electron cloud.
As you might expect, this gets pretty complicated, pretty quickly! Sometimes only one type of crystal structure disturbance is involved (like an impurity atom in a normally colorless material). But a crystal structure may be complex and normally have different kinds of atoms. Myriad impurities might then replace the atoms that are normally there. In some cases a crystal grows with BOTH chemical and structural defects. Any mechanism that produces a visible color in a crystal is called a color center.
And as if this wasn’t complicated enough, MOST crystals have structures that are not equally symmetrical in all directions. In other words, light moves through these materials differently in different directions. As a result their color normally varies with direction, and these colors can be further modified by impurities and defects.
A lot of research has been done on diamond color. It turns out that yellow diamonds are colored by nitrogen atoms in the structure, either singly or in clusters. Blue diamonds are tinted by atoms of boron. Pink diamonds are special, not only in the rarity of their hue, but also in the complexity of the origin of their colors.
The exact mechanism that creates a pink color in diamond is actually not yet fully understood. We know that there is a general absorption of a wide band of light wavelengths, centered around the ones we perceive as yellow. The emerging light therefore is white MINUS the absorbed colors. What comes out is mostly red and purple, but the term ‘pink diamond’ includes a range of hues involving tints of brown, orange and purple.
The color in dark pink diamond is often seen to be concentrated within specific layers, called lamellae. It is believed that these layers are created by shear stresses that push on planes of atoms in the diamond structure, offsetting them due to a kind of slippage. The mechanism involved, as earlier described, is called plastic deformation because the structure itself does not break, but instead stretches itself to accommodate the pressure causing the slippage. The coloration mechanism involves the resulting disruption of electron distribution in the structure, causing the absorption of specific light wavelengths. Some of the disruption in pink diamonds is created by plastic deformation. But research has indicated that complex color centers are also involved, ones that contain several atoms of nitrogen, as well as vacancies.
Several localities have produced pink diamonds. The darkest pinks are the ones that have bands of intense color due to plastic deformation. These are almost uniquely found in the Argyle deposit in Australia. Other localities, including India, Brazil, Borneo and Russia, as well as various places in southern Africa, have yielded pinks and purplish pinks that have a more diffuse and less saturated coloration. These lighter hues are believed to be caused primarily by nitrogen and vacancy-related color centers. Considerable research on diamond color and origin is underway. Scientists hope one day to be able to correlate the coloration of diamonds to their occurrence, and ultimately to the geology of their formation.
ALL gemstones acquire their color by the selective absorption of parts of the white light that surrounds us. ALL gems are therefore ‘colored stones’. A mineral that is intrinsically colorless can remain so ONLY in rare cases. These are instances where a growing crystal remains unaffected by the chemical and physical impurities and disturbances that are ubiquitous in geological environments.
The mechanisms that create color in gems are numerous and complex. The science involved is formidable, but can ultimately be understood in the form of simple models and metaphors that relate to our daily lives. Humans have evolved to ‘see’ only the tiniest part of the sea of radiation that pervades the universe.
But despite that limitation we are indeed fortunate to be able to enjoy the incredible diversity of hues that surrounds us daily in the natural world.
Joel Arem is a former crystallographer and curator at the Smithsonian Institution in Washington, DC. He holds a Masters in Geology and a PhD in Mineralogy from Harvard University, and is a Fellow of the Gemmological Association of Great Britain. He is the author of the Color Encyclopedia of Gemstones (1977, 1987) and 6 other books and numerous articles on minerals, gems and crystals.