In a previous color, we’ve talked about what makes things colored — the way objects absorb, reflect, or transmit light at different wavelengths. But let’s be a bit more specific. What gives gemstones colors, or rather, what changes the way they absorb and reflect light so that they come out in so many different, striking colors?
Ironically enough, in gemstones, color is often influenced by trace impurities or defects in their crystal structures. These which modify how light interacts with the material, creating the rich and varied hues we see.
Impurities and Crystal Structure
At their core, gemstones are minerals, each with a unique chemical composition and crystalline structure. These structures are lattices—essentially three-dimensional grids—formed by repeating patterns of atoms. Pure minerals are often colorless or have a very light hue. It is the presence of impurities, or trace elements, that give gemstones their vivid colors.
Impurities are typically foreign atoms or ions that replace or occupy spaces within the crystal lattice. These substitutions alter how the mineral absorbs and reflects light, resulting in color. Let’s look at some examples.
Amethyst: Iron’s Purple Touch
Amethyst, a type of quartz, owes its captivating purple hue to the presence of iron impurities. When iron (Fe) atoms replace some of the silicon (Si) atoms in quartz’s crystal structure, they introduce what’s known as a “color center.” This color center modifies how the crystal absorbs light, primarily in the yellow and green parts of the spectrum, making the stone appear violet or purple to the human eye (the more one part of the spectrum is absorbed, the more the opposite is reflected).
The depth of amethyst’s color can vary based on the amount of iron present and the specific conditions under which the stone formed. The intensity of the color can also be influenced by exposure to natural radiation over millions of years, which enhances the color centers created by the iron impurities.
Ruby: Chromium’s Crimson Flame
Rubies are a red variety of the mineral corundum, which is essentially aluminum oxide (Al₂O₃). The striking red color of rubies is due to chromium (Cr) ions, which replace a small number of aluminum ions within the crystal structure. When chromium ions are present, they alter the way the crystal absorbs light. Specifically, rubies absorb light in the yellow-green spectrum and reflect red and blue light, with red predominating. This selective absorption and reflection give rubies their characteristic fiery red color.
The intensity of a ruby’s red color can vary based on the concentration of chromium. In fact, the most prized rubies—known as “pigeon’s blood” rubies—are distinguished by their deep, pure red hue, often with a hint of blue.
Emerald: The Green of Beryllium and Chromium
Emeralds, another popular gemstone, derive their lush green color from trace amounts of chromium or sometimes vanadium. Emeralds are a variety of the mineral beryl, which is primarily composed of beryllium aluminum silicate (Be₃Al₂(SiO₃)₆). The substitution of chromium or vanadium for aluminum or silicon ions in the beryl structure alters its optical properties, particularly how it absorbs and reflects light.
Emeralds absorb light in the red and blue regions of the spectrum, reflecting green light, which gives them their vibrant color. Like rubies, the specific shade of green can vary depending on the exact nature and concentration of the impurities.
Sapphire: A Palette of Colors
While many associate sapphires with the color blue, these gems actually come in a rainbow of colors. Sapphires are also a variety of corundum, like rubies. Yes, sapphires and rubies are basically the same minerals, although they look so very different.
The blue color most commonly associated with sapphires is typically due to trace amounts of iron and titanium. When these elements are present in the crystal lattice, they form what is known as “charge transfer,” where electrons move between adjacent iron and titanium ions when the crystal absorbs light. This electron transfer absorbs light in the red and yellow parts of the spectrum, leaving the blue light to be reflected, giving the sapphire its blue color.
However, sapphires can also occur in pink, yellow, green, and other colors depending on the specific impurities present. For instance, the presence of vanadium can produce violet sapphires, while a combination of iron and other elements can result in yellow or green sapphires.
The spectrum of colors available in sapphires is why some gemstones are referred to as “fancy sapphires” when they display colors other than blue.
Diamond: Pure Carbon, But Not Always Colorless
Diamonds are cherished for their brilliance and typically come to mind as colorless gems. However, diamonds can exhibit a range of colors, including yellow, brown, blue, green, pink, and even red. Pure diamonds are made entirely of carbon atoms arranged in a perfect tetrahedral lattice. When diamonds are colorless, it is because there are no significant impurities or defects to disrupt the passage of light through the crystal.
As you may have guessed by now, minerals are rarely truly pure.
Colored diamonds, on the other hand, owe their hues to various impurities or structural anomalies. Yellow diamonds, for instance, get their color from nitrogen atoms that replace some of the carbon atoms in the lattice. When nitrogen is present in pairs or groups within the diamond, it can absorb blue light, giving the stone a yellow appearance.
Blue diamonds, such as the Hope Diamond, owe their color to trace amounts of boron, which absorbs red, yellow, and green wavelengths, reflecting blue. Meanwhile, pink and red diamonds are believed to get their color from structural distortions within the crystal lattice rather than from impurities.
Topaz: A Golden Glow and More
Topaz is a fascinating gemstone that can occur in a range of colors, from the familiar golden-yellow to blue, pink, and even colorless. The color of topaz is often influenced by trace elements and radiation. The golden-yellow hue, which is most commonly associated with topaz, typically results from the presence of chromium or iron impurities.
Blue topaz, which is often found in jewelry, is usually treated with irradiation followed by heat treatment to achieve its vivid blue color. Natural blue topaz is quite rare and usually pale.
Pink and red topaz get their colors from defects in the crystal structure that create color centers. These color centers can trap electrons, which absorb certain wavelengths of light and produce pink or red hues. Heat and irradiation treatments can also enhance or alter the colors of topaz, making it one of the most versatile and widely treated gemstones on the market.
Tourmaline: A Gemstone Rainbow
Tourmaline is one of the most colorful gemstones available, with varieties spanning nearly every hue imaginable. The incredible range of colors in tourmaline is due to its complex chemical composition and the presence of various trace elements. Tourmaline is a boron silicate mineral that includes elements like aluminum, iron, magnesium, sodium, lithium, and sometimes copper. The presence of iron and titanium can give tourmaline shades of blue and green, while manganese can produce pink, red, and sometimes brown colors.
One of the most prized varieties, Paraíba tourmaline, owes its vivid blue and green hues to trace amounts of copper. In some cases, tourmaline can even display multiple colors in a single crystal, known as bi-colored or watermelon tourmaline, where pink and green hues coexist due to variations in trace element concentrations along the crystal’s length. Tourmaline’s wide array of colors and unique ability to display multiple colors simultaneously make it a favorite among collectors and jewelry enthusiasts.
It’s not just them — it’s also us
The colors of gemstones are not solely a result of their chemical makeup. The physics of light and human color perception also play crucial roles. When light enters a gemstone, it can be absorbed, transmitted, or reflected. The wavelengths of light that are absorbed versus those that are reflected determine the color that we perceive.
For instance, a ruby appears red because it absorbs light in the yellow-green part of the spectrum, reflecting red and some blue light. Amethyst, on the other hand, appears purple because it absorbs light in the yellow and green regions, reflecting a mix of blue and red. This interaction between light and the gemstone’s atomic structure is a fundamental aspect of why gemstones have their characteristic colors.
Furthermore, the way a gemstone is cut can enhance or diminish its color. A well-cut gemstone will reflect light internally and back to the viewer’s eye, maximizing its brilliance and apparent color. Conversely, a poorly cut gemstone may appear dull or darker, even if it is of high quality.
Hopefully,
