Colour-changing gemstones are often favourites among jewelry wearers and collectors alike. The play of colour in gems gives the stones depth, radiance, and intrigue, adding that little bit of something extra to the gemstones.
While many people are mesmerized by the play of colour in gems, it’s also common to have questions about the phenomenon. What is the play of colour? Why does it happen? And what are the different types of play of colour in gemstones?
Luckily, there are answers to each of the questions above. If you’re curious about the play of colour in gemstones, here’s what you need to know.
Next, we’ll explore the story of how the play of colour in minerals and gemstones creates iridescent effects through interference.
INTRODUCTION TO GEM COLORS
Gem colours have long captivated our imaginations, drawing us in with their vibrant shades and dazzling brilliance. But what gives gems their remarkable colours? The answer lies in a fascinating interplay between the crystal structure of the mineral, the presence of trace elements, and the interaction of light with the stone. Each gem’s unique structure and chemical composition influence how it absorbs and reflects light, resulting in the rich reds of rubies, the bright blues of sapphires, and the diverse array of hues found in other precious stones. From deep, saturated tones to delicate pastels, the phenomenon of gem colour is shaped by a combination of factors that work together to create the stunning visual effects we admire. By understanding the science behind these colours, we can better appreciate the beauty and individuality of each gem and the remarkable natural processes that bring them to life.
FACTORS INFLUENCING COLOR IN GEMS
A combination of factors determines the colour of a gemstone, each playing a crucial role in the stone's final appearance. At the heart of it all is the crystal structure—the specific arrangement of atoms within the mineral. This structure dictates how light is absorbed, reflected, and transmitted as it passes through the gem. Trace elements, such as chromium, iron, and titanium, play a crucial role. For example, the presence of chromium gives rubies their signature red colour, while iron and titanium can produce a spectrum of colours, including blue, green, and yellow. Similarly, the vibrant green of emeralds is influenced by trace amounts of chromium or vanadium replacing some ions in their crystal structure. The way light strikes the gem—its angle and intensity—further influences the colour we perceive. Certain minerals, like opal and moonstone, are especially known for their play of colours, a result of light diffraction within their unique structures. All these factors—crystal structure, trace elements, and the interaction with light—combine to create the dazzling array of colours seen in gems, making each one a unique work of natural art.
LIGHT AND COLOR: THE SCIENCE BEHIND THE SPECTACLE
The mesmerizing colours we see in gems are the result of a scientific dance between light and the mineral’s crystal structure. When white light, which contains all the colours of the visible spectrum, encounters a gemstone, it can be absorbed, reflected, or transmitted depending on the stone’s crystal lattice and the trace elements present. For instance, the crystal lattice can act like a diffraction grating, separating white light into its component colours—red, orange, yellow, green, blue, and violet. This separation, or diffraction, is what gives certain gems their rainbow-like effects. In some minerals, such as opals and moonstones, the interference of light as it passes through the structure creates a shimmering, iridescent play of colours. The specific wavelengths of light that are absorbed or reflected depend on the elements within the crystal, resulting in the unique spectrum of colours that each gem displays. This interplay of light and structure is what makes gems so visually stunning and endlessly fascinating.
WHAT IS INTERFERENCE AND PLAY OF COLOR IN GEMS?
Play of colour occurs when gemstones appear to have colour-changing properties. Broadly, the play of colour is an optical phenomenon that occurs due to specific characteristics in the stone. It could have to do with a gem’s physical structure or the presence of inclusions, for example.
Different varieties of gemstones display unique play-of-colour effects, and the way these varieties display play of colour can differ based on their structure and composition. With many kinds of the game of colour, diffraction, interference, or both are responsible for the colour-shifting phenomenon you see in certain gems. However, that doesn’t mean each play of a colour type isn’t unique, as they certainly do stand apart from one another.
COLOR CREATION: HOW GEMS PRODUCE PLAY OF COLOR
The play of colour in gems is a truly captivating phenomenon, created by the intricate interaction of light with the mineral’s crystal structure. In gems like opals and moonstones, this effect is produced when light is diffracted as it passes through layers within the crystal. These layers can split light into a spectrum of colours—red, orange, yellow, green, blue, and violet—resulting in the shifting, multicoloured flashes that make these gems so unique. The presence of trace elements such as iron and titanium can further influence how light interacts with the stone, subtly altering the colours that appear. Structural defects, like tiny inclusions or imperfections in the crystal lattice, can also enhance the play of colour by scattering light in different directions. The thickness of the mineral’s layers also plays a role, with thinner layers often producing more vivid and intense colours. All these factors—crystal structure, trace elements, inclusions, and layer thickness—work together to create the remarkable play of colour that makes certain gems so enchanting and sought after.
DIFFERENT TYPES OF PLAY OF COLOR IN GEMS
Adularescence
Adularescence occurs in stones that contain alternating layers of two different types of minerals within the host mineral. The host mineral interacts with these internal layers, causing the light that reaches the depths of the stone to scatter and produce the adularescence effect. This gives the gem a glow that appears to emanate from beneath the stone’s surface. That glow will also appear to float, shifting as the stone is moved.
When it comes to the glow's appearance, it typically ranges from electric blue to milky white. Additionally, it tends to be more pool- or cloud-like.
Like many plays of colour gem phenomena, it’s easier to see adularescence when a stone is cut into a cabochon. The curve of the cut and polish enhances the effect, making it highly visible in gemstones that exhibit this feature.
Adularescence is classically associated with moonstone, where layers of feldspar within the host mineral cause interference, creating a light phenomenon that gives the gem a shifting glow. However, it can also occur in other stones, including rose quartz.
ASTERISM
While the colour-changing aspect of asterism isn’t as strong as you find in some other types of play of colour, it is striking, nonetheless. When light hits the stone with asterism, it bounces off of fibres—specifically fibrous inclusions, growth tubes, or needles—within the gem. Once that happens, the light creates a star pattern, usually featuring four to six rays.
Typically, the star pattern is a lighter hue than the rest of the stone. In some cases, it may be a softer shade of the gem’s main colours. In others, it may be bright white, helping it stand out more against the gemstone’s base colouring.
Asterism can happen with a wide range of gems. Rubies and sapphires are the most popular gems that can produce asterism, but it also occurs in diopsides, emeralds, garnets, moonstones, spinels, and topazes.
While the stone type does play a role in determining whether asterism occurs, the cut also plays a significant role. It is typically only seen in cabochons, not faceted stones.
AVENTURESCENCE
With aventurescence, you get a visual effect that gives the stone a metallic glitter that seems to occur deep within the stone. This happens with mineral platelets that have a specific orientation within the gemstone. When light hits those platelets, which are technically inclusions made of other minerals, they twinkle.
The mineral inclusion type can impart specific colours to the glitter effect. For example, when hematite is present, the twinkle may appear silvery. With copper or pyrite, it has a warmer, golden tone.
Aventurine is one of the most popular gems that exhibits aventurescence. However, it certainly isn’t the only one. Sunstone is also known for the effect, as well as its synthetic variant, goldstone.
CHATOYANCY
Chatoyancy is another word for the “cat’s eye” or “tiger’s eye“ effect caused by fibrous inclusions within the gem. These thin, parallel inclusions are commonly referred to as “silk.”
Tiger's eye is a well-known gemstone that forms when quartz gradually crystallizes around crocidolite fibres. During this process, the fibres become exposed to oxygen, causing the iron within them to oxidize. This reaction imparts the characteristic golden and brown colours to the stone, giving tiger's eye its captivating gold-like lustre and distinctive appearance.
When chatoyancy occurs, you’ll see a near-straight line across the stone that seems to move as you shift the gemstone. It mimics the shape of a cat’s pupils when they become narrow slits.
There can also be a 3D illusion aspect to the phenomenon. With this, the stone appears to have more depth or may seem to glow.
Again, chatoyancy is a phenomenon that’s easiest to see in cabochons, as that shape is best for displaying this form of play of colour. It allows the band of light to shift across the stone’s surface with ease, making it more noticeable.
If the chatoyancy seems to create two zones of colour within the stone, with one side looking lighter than the other, that is called “milk and honey.” The gem isn’t two different shades; it just appears to be that way due to the optical effect.
A wide range of stones can exhibit chatoyancy, including actinolite, apatite, beryl, chrysoberyl, garnet, iolite, kyanite, moonstone, opal, peridot, quartz, spinel, topaz, tourmaline, zircon, and more.
LABRADORESCENCE
Labradorescence is an optical effect associated with labradorite. It references the metallic shimmer often seen in gems, which creates iridescent blues, greens, reds, oranges, and yellows that appear to dance within the stone.
This play of colour occurs because of interference and reflection between layers within the stone. The reflection of light from the twinning surfaces is affected by the thickness and arrangement of these layers, influencing the intensity and range of colours seen in labradorite. Twinning surfaces cause the reflected light to enter different spectrums, creating the illusion of a wide range of hues.
When it comes to cuts, cabochons are the most popular. However, care has to be taken to ensure it’s cut in the right direction. The goal is to achieve full “face-up” colour, where the labradorescence is strongest when the stone’s face is viewed directly. If the wrong angle is chosen, the effect will either diminish or be unobservable.
You’ll also see labradorescence in spectrolite. However, labradorite and spectrolite are essentially the same stone; they are just mined in different places.
OPALESCENCE
As the name suggests, opalescence is the opal's play of colour variation of the colour-changing phenomenon. With opals, the gem is made of layers of silica spheres. The spheres split the light into a full range of colours, creating a shifting platelet look with different segments taking on different hues.
In precious opal, the near rainbow of colours is a hallmark feature. This vibrant play of colours is caused by the regular arrangement of submicroscopic silica layers within the mineral, which interfere with light and produce the striking optical effects. You may see purples, pinks, blues, greens, yellows, oranges, and reds. Additionally, the strength of the colouring can vary from one opal to the next. Some will be incredibly vibrant, while others lean more toward pastels.
Fire opals have a more specific opalescence. In most cases, the dominant colours displayed are limited to flame-like hues, including red, orange, and yellow. However, it can still certainly have purples, blues, and greens; it’s just that the warmer colours usually take center stage.
At times, the colour's appearance is due to the hue of the opal itself. Black opals typically exhibit very strong colours, while white opals, which can actually range from clear to soft gray, may have pastel or vibrant colorations in their opalescence.
It’s important to note that not all opals have opalescence. Common opals may only have a milky sheen, giving them more of an adularescence look.
Read this article to learn more about gemstones that change colour under different lighting conditions.
