The science behind fancy-colored diamonds: why are diamonds colored?
Diamonds are some of the most admired and desired gemstones in the jewelry industry. The ones we’re most familiar with are the colorless diamonds. But did you know that diamonds can have many colors? Depending on how diamonds are formed, they can have different colors. Here, we discuss what gives fancy-colored diamonds their color.
The color of colorless diamonds
Diamonds were created deep beneath the Earth’s crust, in the upper mantle. The high pressure and temperature conditions in the upper mantle pushed and sealed the carbon atoms together to form the diamond crystal structure.
For simplicity, let’s look at the diamond structure in two dimensions, as if looking at it from the side (image below). All the grey atoms are carbon atoms.
If all the positions in the diamond crystal structure were occupied by only by carbon atoms and there were no defects or impurities, then we have the colorless diamonds. However, there is no such thing as a completely perfect diamond with absolutely no impurities. That’s why even some of the colorless diamonds can have a certain subtle color, but the more clear the diamond, the fewer defects it has. Thus, the colorless diamonds can be classified as colorless, near colorless, and faintly colored depending on the coloring present in the diamond. The most pure ones among these are the ones that appear absolutely colorless to the eye.
Depending on where the diamonds were formed and the precise conditions they were subjected to during their formation, they can have different colors. The stronger colored diamonds are called fancy-colored diamonds.
Fancy-colored diamonds—what gives diamonds their color?
There are different factors responsible for the different diamond colors. These include:
The presence of certain impurities, like other chemical elements, in the diamond structure (where another chemical element replaces some of the carbon atoms in the diamond lattice).
The presence of defects in the diamond lattice.
The effects of radiation on diamonds.
The effects of impurities on diamond color
Fancy-yellow diamonds
Diamonds have a yellow color when there are nitrogen atoms present in the diamond crystal structure. We know there are other elements besides carbon atoms that are also present in the Earth’s upper mantle. So when the high pressures and temperatures brought the carbon atoms together, they also trapped some of those other elements in the diamond crystal structure. If there were higher amounts of nitrogen atoms present in the area where the diamonds were formed, then some of those nitrogen atoms were trapped in the diamond structure as the high pressures and temperatures pushed the atoms in their positions in the crystal structure. And it’s those nitrogen impurities that took the place of the carbon atoms in the crystal structure that lead to the yellow color of diamonds.
Fancy orange diamonds
Just like the yellow diamonds, the color of the orange diamonds is given by the presence of nitrogen impurities in the diamond crystal structure. The difference between these two comes from the way in which the nitrogen atoms are organized in the diamond lattice. A higher concentration of isolated nitrogen centers in the diamond lattice can lead to an orange color of diamonds. There could also be other reasons for the orange color of diamonds. One of them is a combination of having a plastic deformation of the crystal and having two nitrogen impurities and a vacancy grouped together in the crystal structure. A vacancy means that one of the carbon atoms is missing from where it’s supposed to be in the lattice.
Fancy blue diamonds
The formation of the blue diamonds is similar to that of the yellow diamonds in the sense that as atoms get packed together in the diamond lattice, some of the carbon atoms are substituted by other chemical elements. And in the case of the blue diamonds, the chemical element that substitutes some of the carbon atoms is boron.
Fancy violet diamonds
The color of the fancy violet diamonds is the result of hydrogen atoms being present in the diamond lattice. Similar to the yellow and blue diamonds, here, some of the carbon atoms in the crystal structure are substituted by other atoms, in this case the impurity being the hydrogen atoms.
There are two more fancy-colored diamonds whose color is given by impurities, but in these two cases, the impurities do not come from the presence of a single chemical element, but from certain mineral inclusions. And these are the white and black diamonds.
Fancy white diamonds
Sometimes the term “white diamonds” is used by people referring to the colorless diamonds. However, fancy white diamonds are not the pure colorless ones, but those that contain some tiny particle inclusions that give the diamond a white opalescent color. While the colors of the previously discussed fancy-colored diamonds are due to atomic-level defects, the defects in fancy white diamonds are due to larger particle inclusions in the nanometer to micrometer size range. The exact chemical composition of these particles is not yet known, but scientists have found that some of these particles contain nitrogen, hydrogen, and even some transition metals, like nickel. These particle inclusions give the fancy white diamonds a milky-like aspect.
Another possible cause for the white color of diamonds is the presence of dislocation loops. These happen when there’s an additional plane of atoms in the crystal structure. These distortions in the crystal structure then lead to the “hazy” aspect of white diamonds.
Fancy black diamonds
Similar to the white diamonds, the color of fancy black diamonds is given by different particle inclusions. But, in the case of the black diamonds, we know what these inclusions are, and they are small particles of graphite, hematite, and pyrite. These mineral inclusions get trapped in the diamond structure during the diamond formation in the Earth’s mantle. These mineral inclusions are uniformly distributed through the diamond structure, making it difficult for the light to pass, giving the diamonds their black color.
The effects of lattice defects on diamond color
By defects, I’m referring to certain distortions in the structural arrangement of atoms in the diamond crystal structure. These are caused by external lateral forces present in the Earth’s mantle during the diamond formation, like the movement of tectonic plates. When these external forces act on the structure, they can induce the displacement of carbon atoms within the diamond lattice.
These small displacements of carbon atoms happen along the glide planes in the diamond structure. These small shifts in the carbon atoms positions along the glide planes are not that big that they would change the entire diamond structure, but they do cause changes in the way light is absorbed and transmitted by the diamond structure. When light passes through these planes of displacement, certain light wavelengths are selectively absorbed and others are selectively transmitted.
Fancy pink diamonds and fancy red diamonds
Glide planes are responsible for several of the fancy colors of diamonds. For example, when these defects cause the diamonds to selectively absorb light of wavelengths corresponding to the green color, then they will transmit, and we will see, a pink or red-colored diamond. Whether it’s a pink diamond or a red diamond depends on the intensity of the transmitted light, which depends on how those glide planes are positioned in the diamond structure.
Fancy brown diamonds and fancy purple diamonds
The fancy brown and purple diamonds also get their colors from the interference of light with the glide planes. However, in the fancy purple diamonds, some of the color might also be due to the nitrogen impurities present in the lattice.
The effect of radiation on diamond color
Fancy green diamonds
Diamonds originated in the Earth’s mantle, but they come from various regions around the globe. Depending on where they come from, they were exposed to different environments as they were formed because of small variations in composition of the Earths’s layers. Some of those variations in compositions included the presence of radioactive minerals close to where the diamonds were formed. Those radioactive minerals emit radiation that can affect the structural integrity of the surrounding materials. Thus, when diamonds are formed in the vicinity of radioactive minerals, the radiation emitted by these minerals is strong enough to remove some of the carbon atoms from their position in the diamond crystal structure. When an atom is displaced from its position in the crystal structure, we are left with a vacancy where the old atom used to be. The presence of these vacancies makes for light of certain wavelengths to be absorbed and others to be transmitted. Our eyes detect the transmitted light, and when the wavelength of the transmitted light corresponds to that of green light, we see the green color. And that is the case of fancy green diamonds.
The rarest fancy color of diamonds
The rarest fancy-colored diamond is the fancy red diamond. The red-colored diamonds are so rare that there are only about 20-30 of them in the whole world. So if you ever get to hold one, remember you’re holding one of the most rare pieced of heritage from our planet.
Let me know in the comments which is your favorite fancy diamond color and why. Mine are the white ones. I think it’s because my favorite stone is opal and the milky aspect of the white diamonds makes them look a bit like opals. Do you like any of the fancy-colored diamonds? Or do you think that the colorless diamonds are nicer? Their purity does make them pretty special.
