A mystery red gemstone is in front of you on a table. Is it a ruby? Is it a garnet? Is it a red diamond?
The DeYoung Red Diamond, held in the Smithsonian’s National Gem Collection in Washington D.C., presents a perplexing case. Observers originally thought the deep red 5.03-carat gem was a garnet. The large stone was set in a pin and purchased by a Boston jeweler at a flea market. The jeweler, S. Sydney DeYoung, noticed that stone held up better over time than a garnet should. After some testing, he realized that his stone was actually a rare red diamond. He extracted it from the pin and willed it to the Smithsonian, where it is now on display.
DeYoung’s story shows how easily a diamond can be mistaken for another gemstone if it isn’t colorless. Traditionally, we think of diamonds as colorless, crystal clear, sparkly gemstones. But most of them aren’t. In fact, the Gemological Institute of America (GIA) classifies diamond colors on a scale from D to Z. Diamonds from D to F are considered colorless, and diamonds at the end of the alphabet have a yellowish hue. The GIA has master color diamonds for each letter and classifies new stones against these standards.
Beyond these faint, yellowish color differences, some diamonds reflect colors across the spectrum. These “fancy diamonds” come not only in yellow, but also brown, red, pink, and plenty of other colors, and they’re quite rare. A colorless diamond is made up entirely of carbon atoms that have come together over millions of years under intense heat and pressure, while fancy colored diamonds take up impurities – non-carbon atoms – during formation. Diamonds form in the presence of other elements, which get incorporated into the diamond’s carbon lattice structure and reflect colors other than white. For example, diamonds with boron atoms woven into their molecular structure have a blue hue. The more boron, the darker the blue. Diamonds with nitrogen impurities appear yellow, and diamonds with defects in their lattice structure appear brown. Occasionally, diamonds form near sources of radiation, which is thought to perturb their structure and give them a greenish hue. The causes of pink and red infiltration into a diamond’s structure remain elusive.
The GIA grades the color saturation of fancy diamonds not with letters but with a more subjective scale: “light, normal, intense, or vivid”. While color reduces the value of diamonds on the D to Z scale, fancy colors are valued more subjectively. When selecting colored diamonds, it’s more important to select a hue that you personally find beautiful than to worry about whether the stone is pure.
In addition to the “D to Z” diamonds and the “fancy colored” diamonds, there is a third color scale for brown diamonds. Brown diamonds are considered fancy if their color is much more saturated than a diamond that registers as Z on the traditional D to Z color scale. Unlike other colored diamonds which contain impurities, the brown color is thought to come from defects in the molecular lattice structure. Until a few decades ago, these diamonds weren’t used in jewelry – they were only used in factories. But recently, Le Vian Jewelers rebranded brown diamonds as “chocolate diamonds,” which have bright brown tones.
Although it could be easy to mistake a red diamond, such as the DeYoung Red Diamond, for a garnet or ruby by eye, the true difference is easy to spot when you zoom all the way in on the stone’s molecular structure. As you now know, a diamond is composed entirely of carbon atoms arranged in a strong tetrahedral structure. Garnets are a totally different story because they involve several different atoms. The general chemical formula for a garnet is X3Y2(SiO4)3, where the X position is filled by ions missing two electrons (divalent cations) such as calcium, manganese, iron, or magnesium. The Y position is filled by ions missing three electrons (trivalent cations) such as aluminum, iron, or chromium. The different combinations of these cations lead to different colors of garnets. The ions in the X and Y positions form an octahedral shape, and the [SiO4]4- ions form a tetrahedral shape. All together, garnets have a combined octahedral/tetrahedral shape. Fun fact: A 2008 study in the Journal of Gemology found that garnets also show some magnetic properties, which helps to identify them.
Rubies, like sapphires, are a type of hexagonal corundum. In fact, rubies and sapphires are really the same gem. The difference is only the amount of chromium impurities that seep into the chemical structure. The chemical formula for a ruby is Al2O3. If the entire ruby is formed from aluminum and oxygen ions, then the stone will be colorless. However, color appears when the position of the aluminum ions is usurped by chromium ions. If less than 1% of the aluminum is replaced chromium, then the stone will be red and called a ruby. The difference between rubies and sapphires kicks in at the 1% chromium threshold. Pink sapphires have slightly more than 1% of chromium, and it can often to be difficult to distinguish them from rubies. If chromium takes over more than 1% of the aluminum spots, then the gem is a sapphire, and it can be blue or many other colors – depending again on the percentage of chromium. Perhaps we should start referring to white sapphires as white rubies!
Getting down to the atomic level of gems to determine their molecular structure helps us learn about their properties, but requires quite a bit of laboratory equipment. One method for determining the molecular structure of a gemstone is called X-ray crystallography. In this technique, researchers send a beam of x-rays through a crystal. The path of the beam is bent when it passes through the crystal, and this results in a unique dot pattern. Each type of crystal gives its own unique dot pattern. Think of the dot pattern as if it were the crystal’s fingerprint. Once you obtain a dot pattern, your next step is to figure how the phase of each x-ray changed when it hit the spot where the dot is. The change in phase will tell you if the x-ray has hit an electron along the way or not. X-ray crystallography tells you where all the electrons are in the crystal. Once you know the number of electrons that are clustered together, you can figure out which elements are in the crystal, and you can determine how they are bonded together. (Fun fact: X-ray crystallography was used to determine the molecular structure of DNA around 1951-1953 by Rosalind Franklin, James Watson, and Francis Crick.)
While diamonds come in many deceiving colors, many other gemstones that we typically associate with colors also come in white/colorless varieties and look much like diamonds. Sapphires, for example, most often come in blue, but they’re also found as colorless stones. Without careful examination, one could easily mistake a white sapphire for a diamond. The untrained eye is unlikely to notice that a white sapphire is cloudier than a diamond and doesn’t split light into rainbows like a prism, as a diamond does. Other stones, like topaz, moissanite, and zircon look so much like diamonds, that they’re often billed as diamond-simulants. These stones are natural like diamonds, but they differ in certain properties, like reflectivity and hardness. They are more affordable than diamonds, however, and can be used as a substitute in jewelry.
Moissanite is a naturally occurring gemstone whose formula is SiC. Henri Moissan was the first person to find moissanite crystals in a crater left by a meteor in Arizona. He initially thought they were diamonds, but later identified them as silicon carbide instead of pure carbon. Moissanite is quite rare and is usually found as an inclusion inside another stone, or as a piece left behind from a meteor. Silicon carbide (moissanite) forms in a complex hexagonal/dihexagonal structure, pictured on the right.
To the non-expert, diamonds and other stones can look so similar that people might not know if the two photos below were switched! Are they diamonds, cubic zirconia, or topaz? Answer: They’re not switched because the ISC’s scientists feel compelled to cite sources correctly.) The true difference in these stones becomes apparent when we zoom into the molecular structure. Upon close examination, we can learn that different gems have different molecular structures, atomic compositions, densities, hardnesses, and refractive indices. Regardless of which type of gem may interest you, make sure you consult with a reputable expert so you know what you are buying.
Visit the Grainger Hall of Gems in the Field Museum of Natural History in Chicago. Take a look at their collection of diamonds and other colorless gems. Can you spot differences in the way they reflect light? Are they cut differently? How do you think the fancy colored diamonds reflect light differently than the colorful ones?