Not all that glitters is gold—some of it is phenomenal
When you think about gemstones, top-of-mind choices include diamonds, or even rubies, sapphires, and emeralds—the “big three” of the jewelry world.
But beyond these sparkly, brilliant gems lies a more mysterious category: phenomenal gemstones. These are stones that exhibit unique and captivating optical effects—special, visual phenomena—caused by the way light interacts with the internal structure of the stone.
The result? Gemstones that can appear magical, almost otherworldly. And in some cases, they are just as rare and hard to come by.
These visual effects each have a name and story. From shifting colors to shimmering glows, here are some of the phenomena and the gemstones that showcase them.
Adularescence
Milky and bluish-white, soft and subtle—reminiscent of a gentle, moonlight glow. That’s adularescence, the signature effect commonly seen in moonstone and rainbow moonstone.
This is produced when light scatters between thin, alternating layers of mineral phases within the gemstone and is best seen on a cabochon cut (a smooth, rounded shape) gemstone. The result? An ethereal glow that appears to float just beneath the surface of the stone, shifting gently with the light.
Asterism
More popularly known as the “star effect,” asterism is a phenomenon that’s caused when light reflects off needle-like inclusions known as “silk”—typically rutile—trapped within the gemstone during its formation. While most famously seen in star sapphires and star rubies, it can also occur in other gems like star garnet, star diopside, and star spinel.
This effect is best captured on a cabochon cut stone, though some poorly cut ones could show a fragmented or even a faint star. And if a gemstone’s star is unusually sharp and symmetrical—yet comes at a suspiciously low price—it may very well be synthetic or lab-grown.
Chatoyancy
Chatoyancy is closely tied to asterism in the sense that it relies on the same light-bouncing effects on needle-like inclusions (usually rutile or hematite). But instead of forming a star, it creates a single, luminous band that glides across the surface of the stone—much like a cat’s eye.
Typically, chatoyancy appears in gemstones like cat’s eye ruby, sapphire, and chrysoberyl, along with quartz varieties like tiger’s eye and hawk’s eye. In some cases, aquamarine and tourmaline can even display the cat’s eye effect.
This phenomenon is best seen on cabochon cuts, and can be well-formed or faint, depending on the gem’s quality. It can even be artificially produced on glass, a common material used to imitate chatoyant gems.
Color change
Color change is a phenomenon where a gemstone appears to shift from one color to another, caused by the way light interacts with its trace elements and unique internal structure. But this change isn’t permament—it’s a response to different lighting conditions, usually towards daylight or incandescent light.
One famous example? Color-change alexandrite. Rare and highly sought after (especially if it has a strong color change), this gemstone appears green or bluish in natural sunlight, then changes into a rich reddish or purplish tone under warm indoor lighting (hence the monicker: “emerald by day, ruby by night”). Other gemstones that can exhibit color change include certain sapphires, garnets, and spinels—and yes, even glass can be engineered to mimic this effect.
Most color-change gemstones are faceted, as the angled cuts enhance the shift in color and allow light to reflect more dynamically. But if the color change appears too pronounced or dramatic, there’s a good chance that the stone is synthetic or possibly an imitation.
Iridescence
Described as an “oil on water” effect, iridescence refers to the phenomenon where light interacts with layers within a gemstone, usually tiny fractures or materials trapped within it during formation. As light enters the stone, it’s reflected and refracted multiple times within these layers, causing it to split into a spectrum of colors. The result? Rainbow colors that shift depending on the viewing angle.
Iridescence appears differently in various gemstones—labradorescence in labradorite, and orient in pearls. But stones that are classified as iridescent include fire agate, mother of pearl, ammolite, and some varieties of treated quartz called “aura quartz.”
Labradorescence
Shimmering, almost metallic, yet vibrant in flashes of rainbow color, labradorescence is a form of iridescence that seems to dance across the surface of the stone. This effect is caused by internal layers within the stone, usually different feldspar minerals, that split light into different colors.
This phenomenon is seen in labradorite—the gemstone after which the effect is named—often displaying vivid blues and greens, but can even show golds, violets, and even reds. While labradorite itself isn’t rare or expensive, stones with strong, vibrant labradorescence are especially sought after.
Orient
Orient describes the soft, luminous sheen of a pearl, hence the monicker “pearl of the orient.” A more muted, but just as alluring type of iridescence, this phenomenon is caused when light interacts with the thin, overlapping layers of nacre that form the pearl.
Take note, though, that not all pearls display orient. It all depends on factors like the thickness of the nacre, the overall quality, and natural body color of the pearl. To see if a pearl displays orient, rotate it under a light source. If there’s a soft play of colors on the surface, the kind seen on soap bubbles or aurora, that’s the orient effect.
Play of color
While seemingly similar to iridescence—since both display dazzling, rainbow-like flashes when shifted around—play of color gets its magic from light that interacts across microscopic silica spheres stacked in a grid-like pattern inside the gemstone. It’s more structured internally, which means that the resulting colors are more vivid and defined. More “fixed,” compared to the shifting, floating sheen of iridescence.
Play of color shows in different ways: pinfire (small, closely-spaced dots), harlequin (broad patches), or even what is described as flame (sweeping bands of color). Opal is a famous gem that displays play of color.
But not all opals are the same. Some display a more vivid play of color, especially if the body is darker—think black opals or fire opals. The contrast allows the color to stand out more, making them highly sought after. On the other hand, some play of color can be faint, patchy, or sparse. Some opals don’t even show play of color.
Just be mindful of opals that display a honeycomb-like pattern—the kind that is too uniform and too structured. Chances are, that’s synthetic.
