What Optical Effects Do Different Gemstones Display: and What Causes Them?
If you’ve ever found yourself doing a double take at a moonstone or feeling drawn to opal jewelry, you might be a fan of colored gemstones with optical effects. In fact, there are more of these mesmerizing gems than you might realize—and many reveal their magic only under special lighting conditions. These effects, known as phenomena, are caused by unique mineral compositions or structural features within the stone, resulting in dazzling displays like color shifts, star patterns, or even a glowing appearance.
Let’s explore the eight most common optical effects found in colored gemstones—you may already have one in your collection without even realizing it.
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Asterism
Asterism is probably the best known and easiest optical effect to view in color gemstones. It creates a star-like pattern on the surface of the gemstone, most often seen in sapphires and rubies with a cabochon cut. This effect is caused by needle-like inclusions that create this star pattern when viewed under a direct light source. It most often shows up in a star with six rays, but some gemstones have been known to have up to 12!
Gemstones with asterism are fairly common and can be affordable depending upon the size, but there are some stones that have fetched quite a high price at auction. The most expensive star ruby ever sold at auction is a 19.53-carat Burmese star ruby, which fetched $1.2 million at Christie’s Hong Kong in 2012. The largest and highest valued star sapphire known to exist is the 1,404 carat Star of Adam, which has an estimated worth of approximately $300 million.
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Chatoyancy
Chatoyancy is similar to asterism, but with a different pattern. It is also known as “cat’s eye” because it looks like the pupil of a cat when viewed under direct lighting. The inclusions within the stone cause a narrow glowing band across its top surface. This effect is most often seen in chrysoberyl, but is sometimes present in tourmaline and tiger’s eye, all of which are cut into cabochons. Gemstones with chatoyancy are not very expensive and can be a nice addition to any jewelry collection.
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Adularescence
Adularescence is present in a fairly popular stone: the moonstone. This optical effect is caused by light present between layers of feldspar within the gemstone and creates a soft sheen that gives it a cosmic appearance. The stone appears to glow as it is exposed to different light sources and can reflect different colors depending upon the stone. Moonstones can be cut into cabochons or faceted and are not often expensive. The largest moonstone ever sold at auction is a 50 carat Sri Lankan moonstone, which sold at Christie’s in 2013 for $496,000.
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Labradorescence
Not surprisingly, labradorites most often exhibit labradorescence. This optical effect occurs when light is interfered with as it's passing through the layers of the stone. It can create colorful flashes that resemble the northern lights in shades of green, purple, and blue. Labradorite is also a fairly affordable gemstone, but there are records of some that have fetched high prices at auction due to their unusual labradorescence. The most expensive labradorite ever sold at auction was a 1,400-pound boulder from Madagascar, which fetched $600,000 in 2014. Its striking labradorescence displayed vivid flashes of blue, green, and even gold.
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Iridescence
The cause of iridescence in gemstones is similar to labradorescence; when light reflects between the layers inside the gemstone, colors become visible. But unlike labradorescence, iridescence is more subtle and looks more like a light rainbow effect on the surface of the stone. Some gemstones that show iridescence are ammolite and fire agate, both of which are fairly common–though not inexpensive. Opals also have iridescence, but the effect is called play-of-color when specifically found in opals. The most expensive stone displaying iridescence sold at auction is the Australian Virgin Rainbow Opal that was sold in 2015 for $1 million.
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Play-of-Color
Opals come in many different types and are famous for their play-of-color, which is when bright colors shift on the surface of the stone. This effect is caused when light is refracted within the tight silica structures inside the stone, causing a range of colors to appear. One of the most popular types of opals is the Mexican fire opal, which can range in color from fiery reds and oranges to translucent yellows and clear, often with flashes of green, blue, or purple. They are most often cut in freeform shapes, but can also be faceted, both of which make beautiful, bright jewelry. They are fairly soft, which requires careful handling and storage to preserve their integrity over time.
Australian opals, which make up over 95% of the world's opal supply, are available in many different types, including black, white, boulder, and crystal opals. They have incredible play-of-color and are most often cut into cabochons and used for all types of jewelry. Also quite delicate, Australian opals need to be treated with care to avoid damage. The most popular type of Australian opal is the black opal because the dark color allows for the bright play-of-color to be more readily visible.
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Color Change
Alexandrite displays a color change that is quite dramatic, making it the most popular color changing gemstone, as it changes from emerald green in daylight to a rich reddish-purple under incandescent light. This optical effect is caused by the presence of chromium in the crystal structure, which reacts differently under various lighting conditions. The color change happens when the stone absorbs and reflects different wavelengths of light depending on the environment.
Alexandrite was originally discovered in 1830 in Russia’s Ural Mountains, but today it is mined in several countries including Brazil, Sri Lanka, India, Madagascar, and Zimbabwe.
There are other gemstones that display color change properties such as sapphire and garnet, but they are not as common or distinctive.
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Fluorescence
The last type of optical effect we will discuss is fluorescence, which is only visible under UV light and most prevalent in diamonds, which can glow blue. Fluorescence is not typically visible under normal lighting conditions, as jewelry is rarely exposed to UV light, but it adds a distinctive touch that makes your stone stand out.
Some people search out diamonds with fluorescence for exactly that reason, while others prefer to avoid it. It’s all about personal preference.
Final Thoughts
Optical effects in colored gemstones add an extra layer of intrigue to the already captivating world of gemstones. From the shimmer of adularescence in moonstone to the vibrant flashes of play-of-color in opals, these effects occur due to the way light interacts with minerals and structure within each gemstone.
Understanding these optical effects can assist when you’re looking to purchase a color gemstone, or just help you better appreciate beautiful stones you happen to see. The presence of these phenomenal effects can completely change the look of a piece of jewelry, making it a conversation piece and a cherished design in any collection.