A large green opal with multicolored flashes set in a white gold ring.

Stars & Shimmers: Optical Phenomena in Precious Gemstones

A large green opal with multicolored flashes set in a white gold ring.

There’s nothing quite like the brilliant fire of a perfectly cut diamond—but when it comes to precious gems, “sparkle” isn’t the only visual element that can charm a jewelry lover.  

Many gemstones exhibit uniquely captivating patterns or effects known as “optical phenomena”—essentially, the way light interacts with the structural features or inclusions of the stone. Aside from being visually mesmerizing, these phenomena illustrate the incredible complexity and beauty that can be found in nature.

Here are a few of the major optical phenomena that make certain gems so magical and prized! 

Play-of-Color

“Play of color” is an effect that is commonly found in opals. It describes the mesmerizing, multicolored flashes that appear to dance across the surface of the stone when viewed from different angles.  This display is sometimes referred to as opalescence.

The “play of color” we see in a high-quality opal is produced by the silica structure of the gemstone, comprised of millions of tiny spheres of hard silica layered on top of each other. When light hits these spheres, it is reflected at different angles and directions, creating the dreamlike, dazzling display of color that is so beloved by gemologists around the world. When the size and spacing of the silica spheres are relatively small, refracted blue-green colors are prevalent; when relatively larger, yellow-orange-red colors are seen.

Typically, the more vivid and intense the colors, the more valuable the opal. Black opals, with their darker body tone, are particularly prized for their kaleidoscopic opalescence.  Harlequin opals, in which the play of color appears as a striking checkerboard pattern of diamonds or squares, are also extremely valuable

Adularescence

If you’ve ever been entranced by the soft, shimmering light exhibited by a moonstone, you’ve seen the effects of adularescence in action!

Adularescence is an optical phenomenon that describes the ethereal, lustrous glow found within certain gems. It’s characterized by a ghostly, bluish-white light that appears to float just beneath the surface of the stone—almost like moonlight on water. Most associated with the moonstone, this effect is caused by the scattering of light between the layers of feldspar within the stone.

Moonstones are usually cut into cabochon with smooth, rounded tops in an effort to best display their adularescent properties. A superior stone will also have very few visible inclusions to distract from this effect.

Asterism (Star Effect)

While rubies and sapphires have long been prized for their rich color, a small subset of these stones exhibit a unique visual effect known as asterism.

Asterism is the star-like pattern that appears on the surface of some gemstones when illuminated by a direct light source; it’s caused by the reflection of light from needle-like rutile or hematite inclusions within the stone. The stars will typically have six rays or four rays (or, rarely, twelve rays), and a well-defined, centered star is the most desirable.

This celestial-like phenomenon is best seen in gemstones cut into cabochon. Fun fact: When rubies or sapphires show this phenomenon, they’re known as star rubies or star sapphires!

Art Deco Platinum Star Sapphire & Diamond Ring

Color Change

Have you ever noticed that some gems can change hues depending on the light they are viewed under? This fascinating phenomenon, known as “color change,” occurs in a select few stones containing specific elements and minerals. Though other stones (like garnet and spinel) can display it, the phenomenon is so frequently associated with alexandrite that it’s often called the “alexandrite effect.”

Named after the Russian Tsar Alexander II, alexandrite is renowned for its remarkable ability to transform from a deep green in daylight to a vibrant red under incandescent light. This color change is due to the presence of chromium, a trace element that absorbs and reflects light differently under various lighting conditions.

Color-changing gemstones are highly sought after by collectors and jewelry enthusiasts for their unique beauty and versatility. Their ability to adapt to different lighting environments adds a touch of mystery to any piece of jewelry.

While we think ALL gems are beautiful in their own way, the display of optical phenomena in many gems adds a certain amount of charm and allure. If you’re looking for a truly one-of-a-kind piece of jewelry, you can’t go wrong with a stunning opal, shimmering moonstone, or celestial star ruby.