Have you ever wondered why rubies are red, emeralds are green, and why there are white diamonds, but also pink and yellow ones? What we see as color is actually wavelengths of visible light reflected back at our eye. The color of a gemstone doesn’t emanate by itself, rather color comes from the interaction of light energy with trace elements within the gemstone. Gemstones get their color from various factors, including the presence of certain impurities, their crystal structure and the way light interacts with the stone. Here we offer a glimpse into the world of gemstones and what gives them those beautiful colors that we find so captivating.
Why Are Gemstones Different Colors?
We may think of color as an inherent property of an object like its size or shape, but color has no independent reality. It’s registered through our sensory eyes and brain, and when we describe a color, we’re actually describing an interpretation of our own senses, not the object’s atoms. Most things get their color based on how they react with light. Light consists of wavelengths of individual colors. Visible or “white light” is a combination of color wavelengths that blend to form white. All solids, including gemstones, separate the colors in light. Colors that aren’t absorbed are reflected back, and those reflections are the colors that we see.
As revealed by rainbows, white light is composed of the individual colors of the spectrum: blue, green, orange, red, violet and yellow. When white light passes through a gem, some spectral colors are absorbed. If all spectral colors pass through a gem it appears colorless, if all are absorbed it will appear black. If a gem absorbs all wavelengths except those in the red part of the spectrum, the gem will appear red. Each gemstone will absorb or reflect different color wavelengths depending primarily on the chemicals it contains.
Simplistically, gemstone colors occurs in two ways:
- Self Colored: gemstones said to be idiochromatic or “self-colored” absorb certain wavelengths of light due to characteristics of their chemical structure; the color is actually in the stone. Peridot for example, will always be shades of green due to its high iron content.
- Other Colored: most gemstones are allochromatic, meaning they are colored by impurities or trace elements that aren’t part of their normal crystal structure. For example, pure corundum is colorless, but when traces of chromium are present it becomes red ruby, and with traces of titanium, it becomes blue sapphire.
Impurities change the color of the whole stone, inclusions change only a part of its appearance. Inclusions are large amounts of a foreign substance embedded in the gem. Because the inclusion has a different chemical composition, it absorbs and reflects different color wavelengths. Although most inclusions are undesirable, some can create effects like chatoyancy (cat’s-eyes) and asterism (star- stones). While most colorful opaque minerals get their color from a simple act of an included transition metal element, the colors we see are often because of much more complicated changes to the crystal’s atomic structure.
Can You Change the Color of a Gemstone?
The most common trace elements in gemstones that influence their color are: beryllium (emerald), chromium (emerald, jade, chrome tourmaline, ruby and topaz), copper (Paraiba tourmaline, turquoise and malachite), iron (yellow sapphire, aquamarine, green tourmaline, chrysoberyl, citrine and jade), lithium (green and pink tourmaline), manganese (pink tourmaline, morganite, kunzite and spessartite garnet), sulfur (lapis lazuli), titanium (sapphire) and vanadium (emerald, alexandrite and color-change sapphire).
In short, there’s no way to alter the color of a gemstone. Adding or changing the color would involve changing the way light is reflected, so you’d have to change the gemstone’s entire molecular makeup. Enhancing and improving the colors of some gemstones however is possible through several means including: heat treatment, radiation, oiling and filling.
- Heat treatment is a common practice to enhance or alter the color of certain gems. Heating changes the arrangement of its atoms and consequently its color and has been done for centuries. Heat can lighten, darken, deepen or change a gemstone’s color entirely. It can eliminate inclusions or it can form them, it depends on a number of factors including pressure, temperature, rate of heating and cooling and chemistry. While heat-treated sapphires can achieve more vibrant hues, heat-treated amethysts turn yellow or orange; not all gems can undergo this treatment.
- Irradiation is the process of using electromagnetic radiation to alter a gem’s color, and has been used since the early 1900’s. It’s routinely used to color a number of gemstones including diamonds, beryl, pearls, topaz, yellow sapphire, amethyst and tourmaline.
- Waxing or oiling is often used on emeralds that contain natural fissures that are sometimes filled with wax or oil to disguise them. This wax or oil is also colored to make the emerald appear of better color as well as clarity. Turquoise is also commonly treated in a similar manner.
A large percentage of gemstones on the market are treated in some way to increase their color, clarity or durability. Want to learn more about colored gemstones and designing a stunning custom piece of jewelry? Give us a call at 303-385-8449 or book an appointment!