Above we mention the role that faceting plays in revealing the beauty of a gem. However, the single most significant factor may well be the role that light plays in exhibiting the color of a gemstone. In fact, gems are cut and faceted in the shape and styles that the cutter believes will achieve the best results from the stone’s interaction with light in showcasing its attributes. Many factors influence the cutter’s decision, including the type of gem, shape and size of rough, etc.
The type of lighting used to illuminate a gem can affect the color and quality of color we see. Some consider natural daylight as a standard. However, daylight can vary depending on geographic location and time of day. A stone viewed in natural daylight in one location may look different when viewed in daylight in a different geographic location.
White light is critical in unlocking the colors we see. Light is a form of energy. White light is actually composed of individual colors ranging from violet through blue, green, yellow orange and red. When these different wavelengths are present in equal amounts, our eyes perceive it as being “white”. But if an object absorbs certain wavelengths more than others (selective absorption), the result is color. If one passes white light through a colored object, like a gemstone, any portions of the spectrum that are missing represent areas of absorption. It is these areas of absorption that result in visible color perceived by the eye. The primary causes of the color we see in gems can typically be attributed to impurities, charge transfer, or color centers. There are also other causes but for the purpose of this cursory explanation of the topic only these three common causes are discussed.
Impurities (trace elements not found in the pure mineral form) contained in a gemstone are responsible for the color we see. For example corundum is the species of mineral to which ruby and sapphire belong. In its pure form, corundum is colorless. However, when chromium impurities are trapped in the corundum during crystal formation the crystal exhibits a red color. The strength of color is dependent on the amount of the impurity(s) present. The same impurity will have different effects on different minerals. Chromium creates red in corundum (ruby) but green in beryl (emerald).
For a charge transfer to occur, a compound must have at least two elements in different and variable oxidation states. If this condition is present, it is possible for electrons to be swapped between two elements as energy from visible light is absorbed and transfers electrons from one atom to another. This can result in color formation of alternation.
Color centers are imperfections in the crystal lattice that cause color by absorption of light. The defects are usually the result of radiation damage. Many gemstones will exhibit different appearances when viewed in fluorescent and incandescent light. The energy wavelengths associated with these light sources differ from one another. In certain gems, by changing the light source, one can actually change the color of the gem. Alexandrite is an example of a gem that changes color from incandescent to fluorescent light. Many jewelry stores have bright halogen lights over the jewelry showcases. The intensity of these lights brings out the beauty of the gemstone. But if you would like to know what a gemstone is really going to look like in normal viewing conditions observe it away from the case where lighting is more subdued.