Optical Phenomena in Gemstones
Sometimes the most interesting thing about a gemstone isn’t its color—it’s what happens when the stone interacts with light.
A bright line may move across a cabochon as it turns. A star can suddenly appear under a pinpoint light. Colors may flash from inside a stone, shimmer across its surface, or even seem to change under different types of lighting.
These effects are known as optical phenomena.
Understanding why they occur makes beautiful gemstones even more interesting—and for a lapidary, understanding the phenomenon can determine how a stone should be cut.
Optical Phenomena in Gemstones — Learning Series 004 (PDF)
More Than Just Color
The ordinary body color of a gemstone usually comes from its chemical composition, trace elements, defects in its crystal structure, inclusions, or combinations of these factors.
Optical phenomena are different.
They result from the way light interacts with a gemstone's internal structure, layers, inclusions, fractures, or other features.
Some effects depend strongly on viewing angle. Others require a particular type or direction of light. Many can be dramatically strengthened—or almost completely lost—depending on how the stone is oriented and cut.
Labradorescence
Labradorescence is the colorful flash associated with labradorite.
Light interacts with extremely fine internal structures in the feldspar, producing flashes that may include blue, green, yellow, orange, red, purple, or combinations of colors.
Turn the stone away from the correct angle and the color can disappear.
Chatoyancy — The Cat’s-Eye Effect
Chatoyancy produces a bright band of reflected light that seems to move across the surface of a stone as either the stone or the light source moves.
The name comes from the French expression for cat’s eye.
The effect can be caused by aligned fibers, needles, tubes, or other internal features that reflect light in a common direction.
Proper orientation is critical when cutting a chatoyant cabochon. Cut it correctly and the eye can be sharp and centered. Cut it incorrectly and the effect may be weak or disappear.
Asterism — Stars in Stone
When similar reflective features are arranged in multiple directions, they can produce intersecting bands of light that form a star.
This phenomenon is called asterism.
Star sapphires and star rubies are famous examples, although asterism occurs in other materials as well.
The number and sharpness of the rays depend on the internal structure of the stone and its orientation. These stones are generally cut as cabochons so the star appears properly positioned on the dome.
Adularescence
The soft, floating glow associated with fine moonstone is called adularescence.
Instead of the stronger colorful flash commonly associated with labradorite, adularescence often appears as a softer light that seems to move within or just beneath the surface of the stone.
Moonstone and labradorite are both feldspars, but adularescence and labradorescence are not the same phenomenon.
Aventurescence
Some gemstones contain numerous reflective mineral inclusions that create a sparkling or glittering appearance.
This effect is called aventurescence.
Sunstone is a familiar example. Tiny reflective inclusions can produce flashes or a metallic-looking sparkle as the stone moves in the light.
The word aventurine is related to this phenomenon, although gemstone names and optical-effect terminology are not always used consistently in the marketplace.
Play-of-Color
Precious opal can display shifting patches of spectral color known as play-of-color.
The effect is produced when light interacts with the microscopic internal structure of the opal. As the viewing angle changes, different colors may appear, disappear, or move across the stone.
Play-of-color is different from the ordinary body color of an opal.
Iridescence
Iridescence is a display of changing colors produced when light interacts with very thin layers or structures.
The colors can shift as the viewing angle changes.
Iridescent effects occur in several natural materials and can sometimes resemble other optical phenomena, which is one reason understanding the terminology is useful.
Color Change
Some gemstones appear to change color when viewed under different types of illumination.
This is known as color change.
Rather than simply turning the stone to a different angle, the effect depends on differences in the wavelengths present in the light source and how the gemstone absorbs and transmits that light.
Alexandrite is the classic example, but color change occurs in several other gemstones.
Why Optical Phenomena Matter to Lapidaries
For a lapidary, finding an optical phenomenon is only the beginning.
The rough must often be studied carefully before cutting. The orientation of fibers, inclusions, layers, crystal structures, or color-producing features can determine where the face of the finished stone needs to be.
The shape and height of a cabochon can matter too.
A potentially spectacular piece of rough can become an ordinary-looking cab if it is oriented incorrectly. A carefully oriented cut can reveal an effect that was barely noticeable in the original material.
In these stones, the cutter isn't creating the phenomenon—but the cutter can determine whether you ever get to see it.
Learning more about Optical Phenomena in Gemstones
Learn more in Learning Series 004 — Optical Phenomena in Gemstones.
The guide introduces several of the most important optical effects found in gemstones, explains what causes them, and shows why light, viewing angle, internal structure, and cutting orientation matter.
Optical Phenomena in Gemstones — Learning Series 004 (PDF)
Look at the Stone Again
When examining a gemstone, don't just look at its color.
Move it. Turn it. Change the direction of the light. Try a pinpoint light source. Look at it from several angles.
Sometimes a gemstone has been showing you only part of the story.
The stone provides the structure. Light provides the show.