The inner life of a gemstone is very lively, telling you a lot about it and some gems have fascinating optical effects that are especially intriguing, such as stars or color changes. Known as phenomenal gems, these stones produce a stunning light show, like none you’ve ever seen before. There are several special optical effects — adularescence, asterism, chatoyancy, color change and play of color — some are found in a variety of gems, while others are exclusive to a specific stone.
Adularescence
When you look at a moonstone, you’ll see a sheen of glowing light floating across the gem that is known as adularescence. Moonstone, part of the feldspar mineral family, is made up of very thin layers of orthoclase and albite minerals that alternate throughout the gem. Adularescence is created when light hits the stone, falls between the layers and scatters creating a mystical color that floats across the gem like the full moon on a misty night. Top quality moonstones have a blue sheen against a colorless background. The sheen can also be silver, white or rainbow. It is the adularescence that makes moonstone one of the phenomenal gemstones.
Asterism
When a star appears in a gemstone it is known as asterism. Sapphires and rubies both exhibit asterism in the form of star sapphire or star ruby. The gem must be cut as cabochon (an unfaceted stone that is shaped into a smooth dome on top with a flat bottom) for the star to emerge. The star comes about when very fine, but dense, inclusions in parallel form reflect light inside the stone. Generally, the stars display as six intersecting rays and will be more apparent in some stones than others. Asterism can also be found, very rarely, in garnet, rose quartz, rutilated quartz, spinel and diopside.
Chatoyancy
Chatoyancy, from the French phrase, “ceil de chat”, which translates to cat’s eye, is created by bands of light reflecting from tiny needle-like inclusions in the stone. Chatoyancy, also called chatoyance, forms in a narrow band that moves as the gem is turned or tilted and is more obvious in some stones than others. Chatoyancy may be found in chrysoberyl, often referred to as cat’s eye, jade and occasionally in other gems.
Color Change
Color change gemstones are very rare. These stones will visibly change color when viewed under different light sources, such as daylight or incandescent light. Alexandrite is one of the best known color change gemstones, generally changing from green in daylight to red in lamplight leading to the descriptive phrase “emerald by day and ruby by night”. Very occasionally, a sapphire, garnet or diamond, known as a chameleon diamond, will display color change.
Play of Color
When you see the shifting colors in an opal, those flashes of color are known as play-of-color. Tiny spheres of silica in a grid-like structure allow light to flow between the spheres creating play-of-color and making opal part of the phenomenal gemstone family. The silica spheres can be different sizes and it is the size of the sphere that establishes the colors seen in play of color. There are two types of opal: Precious and common. Only precious opals will have play-of-color.
Mother Nature’s magical light show is captured in beautifully unique phenomenal gemstones, creating unusual jewelry for those who dare to be different.
Top of page: Cat’s eye, diamond and 18-karat gold ring, circa 1950s.
Victorian diamond, moonstone, natural pearl, silver and 18-karat gold pendant; Art Deco diamond, blue star sapphire and platinum ring; Cat’s eye and 18-karat gold ring; Alexandrite weighing 26.75-carats, Photographed by David Weinberg for Alexandrite.net and released to the public domain, courtesy WikiCommons; Art Nouveau diamond, opal, enamel, 18-karat gold and platinum pendant.
Authored by Amber Michelle