Published: June 2013
Adularescence is an optical phenomenon that manifests as a soft shimmer of light that moves within a gemstone as it is rolled back and forth. This property is most commonly associated with moonstone, a member of the feldspar family.
The term cat’s eye is used to describe an exotic optical property that is rarely seen in many gemstones. The effect, when present, appears as a bright, narrow slit – similar to what you see in the eyes of your favorite feline pet. This phenomenon is caused by parallel fibrous or needle-like inclusions. The inclusions interfere with the passage of light, which is scattered and reflected back to the viewer. The effect is best seen on gems cut en cabochon (a dome-shaped style lacking facets). When used by itself, the term cat’s eye always refers to the chatoyant variety of the mineral chrysoberyl. Any other gemstone exhibiting this optical property must have its name specified: quartz cat’s eye, or cat’s-eye quartz, for example.
Color change is a special optical property referred to as a phenomenon. It is a change in hue that occurs when a gemstone is exposed to different lighting environments. The best-known example is alexandrite, the color-change variety of chrysoberyl. When seen in daylight, it appears greenish, but under incandescent light sources, reddish.
Color shift is similar to color change, but more limited in scope. When exposed to different lighting environments, some gems will exhibit a small degree of change, generally within two adjacent colors of the visible light spectrum. The term color shift is used to describe this effect.
Fluorescence is an optical property associated with many gemstones. It occurs when various forms of electromagnetic energy (ultraviolet light, infrared light, or x-rays, for example) are absorbed by the host material, and some portion is re-emitted in the visible-light spectrum. Fluorescence is often used as a diagnostic tool when identifying gemstones.
Iridescence is an interference phenomenon that produces a prismatic, rainbow-like play-of-colors within, or on the surface of a gemstone. The effect may be caused by plate-like inclusions or mineral layers of differing refractive index. Examples include fire agate and rainbow andradite. Iridescence (from the Latin iris meaning "rainbow") is the rainbow of colors reflected from the surface of a gemstone, such as fire agate or andradite garnet.
Opal displays a burst of striking colors known as play-of-color. As the stone is moved, its appearance changes and a sparkling display of rainbow-like colors can be seen from different angles. This play-of-color is caused by the diffraction of light hitting the stone. In the 1960s, intensive microscopes magnifying between 20,000x and 40,000x revealed that opals are made up of tiny silica spheres (150 to 300 nanometers) interspersed with water. The shape, size, and alignment of these spheres affect the color of the opal.
A star gemstone exhibits a star like pattern created when light encounters parallel, fibrous or needle like inclusions within a gem. Light that strikes the inclusions within the gem reflects off of the inclusions, creating a narrow band or line. When two or more intersecting bands of light appear, a star pattern is formed. Depending on the structure of the gem there may be four, six, or less commonly twelve rays. When only one band forms it is classified as a different phenomenon called “cat’s-eye”. Gemstones exhibiting this effect are called “star gems” and are described as “asteriated.”
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