How Gemstones Get Their Colors

My cats like to participate in most of my activities. Their "assistance" generally involves batting supplies off my desk. When they sit and stare at me, it's easy to see how one of my favorite gemstones, cat's-eye chrysoberyl, got its name. The stone is the same golden-honey color as my cats' eyes, and the long, narrow band of light across the middle of it looks like their narrowed pupils.

Cat's-eyes and star-stones are popular names for the special effects caused by light reflections in certain gemstones. They can usually only be seen when the gemstone is cut in a rounded or domed shape (cabochon). The reflections that cause cat's-eyes and star-stones are one way in which color is displayed in gemstones.

Color in gemstones, as it does in all other substances, starts with light. Visible light, the light that humans see with the naked eye, is only a small part of the total light available. Visible light is also called "white light," because it looks white to us. All light consists of wavelengths of individual colors. Visible light is a combination of color wavelengths that blend to form white. You can easily see all the colors in white light when moisture in the air separates sunlight into a rainbow after a rain.

All solids, including gemstones, separate the colors in light. Nearly all substances absorb some color wavelengths. The colors that they do not absorb are reflected back. Those reflections are the colors that we see. Each gemstone will absorb or reflect different color wavelengths depending primarily on the chemicals it contains. For example, peridot absorbs all but the green color wavelengths. The green is reflected back, making peridot look green to us. Rubies absorb everything except red and some blue wavelengths. The red and unabsorbed blue are reflected back to us as a deep red color. Gemstones appear black when they absorb all color. Some gemstones in their chemically pure state don't absorb any color wavelengths. Beryl is one example. Because all the colors are reflected back, the gemstone is the same color as the light striking it. That makes it appear colorless to us. However, tiny amounts of impurities in a stone can cause some light to be absorbed rather than reflected. Depending on the impurity in beryl, we see green (emerald), blue (aquamarine), yellow (heliodor or golden beryl), or pink (morganite).

While impurities can change the color of the whole gemstone, as with the beryls, inclusions change only a part of a stone's appearance. Inclusions are relatively large amounts of some foreign substance embedded in the gemstone. Because the inclusion has a different chemical composition than the gemstone itself, it absorbs and reflects different color wavelengths than the gemstone does. Although most inclusions are undesirable, some can create effects like chatoyancy (cat's-eyes) and asterism (star- stones).

Chatoyancy, which comes from the French words for cat and eye, is a band of light across a gemstone. It is not on the surface of the stone, but comes from within and looks exactly like a cat's eye. The inclusions that typically cause chatoyancy are tiny, needle-shaped crystals of rutile. Rutile is a colorless mineral that reflects light the way diamonds do. To reflect a good cat's-eye, the crystals must be lined up parallel to each other. Cat's eyes occur frequently in chrysoberyl, but are also found in tourmaline, rubies, sapphires, garnets, spinel, and quartz.

If the inclusions are lined up in bands in more than one direction, with the bands all crossing one another at a middle point, the reflected light forms a star. Stars usually have either four or six rays. Stars, or asterism, are found primarily in rubies and sapphires. Quartz and garnet may also sometimes have stars. Random patterns of crystal inclusions may reflect spangles or points of light.

Tiger's-eye starts as the mineral crocidolite, which contains long, asbestos-like fibers. Over time, nature replaces the fibers with quartz crystals and the color gradually changes from blue to yellow and brown stripes. Tiger's-eye gets its name from the chatoyancy visible when it is properly cut.

Color is what makes many gemstones valuable, and light is the source of that color. Chatoyancy (cat's-eyes) and asterism (star-stones) are special reflections of light that add to the beauty and pleasure of the gemstones we prize