Chatoyancy or chatoyance (pronounced shə-TOY-ən-see) in gemology is a mystical optical phenomenon also known as the cat’s eye effect displayed by some stones (minerals, gemstones, or rocks) correctly cut in cabochon.
What is it? Chatoyancy describes the reflection of one or more movable, concentrated, narrow, single sharp, luminous, or bright bands of light with a silky luster that seems to move on the polished curved surface of a cabochon as you turn the gems or source of light.
Chatoyancy occurs when gemstones with fibrous structures or numerous elongated parallel minute fibers, cavities, channels, fluid-filled tubes, or inclusions reflect light perpendicular to their orientation. For instance, tiger eye quartz has a fibrous structure, while cat’s eye chrysoberyl has fibrous inclusions or cavities.
Some minerals or gemstones that show the cat’s eye effect include chrysoberyl, quartz, aquamarine, apatite, ruby, sapphire, tourmaline, alexandrite, and moonstone, among many others.
Learn about chatoyancy, its meaning or definition, its causes, and some minerals that show this optical phenomenon. We will not discuss chatoyancy in wood as it is unrelated to mineralogy.
What is chatoyancy?
Chatoyancy is an optical phenomenon or effect in some appropriately cut gemstones, crystals, or minerals cabochons represented by one or more movable, wavy, or silky narrow concentrated bands of reflected light or sheen that seems to move from beneath their surface as you turn the crystals/gems/minerals or change the angle of incident light.
This single, sharp, bright band or streak of reflected light or sheen resembles the gleaming vertical slit observed in a cat’s eyes or the band of light that nylon or silk thread reel reflects. It occurs in some correctly cut chatoyant gemstone cabochons shown by a single strong light source. Moving the gemstone cabochon or light source will make the reflected streak move on the curved surface of the cabochons.
Minerals or gemstones that show this unusual optical phenomenon are said to be chatoyant. Also, you can use the suffix or prefix cat’s eye to the name gemstone name to mean those that display optical phenomena.
The word chatoyancy originates from the French word chat oeil (or oeil-de-chat), which means cat’s eye or possibly from chatoyer implies to shimmer or shine like a cat’s eye, a characteristic appearance of minerals with this property.
Please don’t confuse chatoyance with luster, as the latter talks about how minerals, rocks, or crystals reflect light or appear under reflected light, i.e., a measure of their surface’s reflectance. Luster can be metallic or non-metallic (adamantine, resinous, vitreous, pearly, waxy, greasy, or dull).
In contrast, chatoyancy is an unusual optical phenomenon where minerals or gemstones reflect a concentrated band of light from beneath their surface that seems to move across the summit of cabochon cut specimens as you move the gems or source of light.
Besides minerals/gemstones, wood may also display a cat’s eye effect. Also, some materials, including glass, may artificially be made to imitate this unique optical phenomenon.
What causes chatoyancy in gemstones?
The chatoyancy effect in gems results from light reflection by minerals with 1) fibrous structure or 2) numerous minute elongated parallel cavities, channels, or fluid-filled tubes or needle-like inclusions like rutile or hematite arranged along the same direction, plane, or orientation inside the crystal lattice. If oriented in three different directions, the phenomenon is known as asterism, closely related to chatoyancy.
This fibrous structure parallel inclusions or cavities will scatter or reflect light perpendicular to their surface, resulting in a luminous streak or band of light. Thus, the moving, wavering reflected band of light is always perpendicular to their orientation.
Chatoyancy is seen in well-cut, polished cabochon gems, i.e., cut en cabochon and in a way that the orientation of the minute parallel fibers, cavities, tubes, or needle-like inclusions follows the gems’ symmetry and parallel to the base of the gemstone.
The reflected light depends on the cabochon’s density and thickness and the orientation of the parallel fibers, cavities, tubes, or needle-like inclusions. Thus, you may get varying degrees of reflected light in different chatoyant gems.
How to cut chatoyant stone
The gems are cut en cabochon to display a cat’s eye effect optimally. i.e., with a convex top and flat base without facets. Also, the cabochon base should run parallel to the orientation or plane of these fibers or inclusions.
Also, you need to consider the gemstone’s symmetry and ensure the bright or luminous streak runs along the long direction of your gem unless you have a round cabochon for the most alluring looks. Your cabochon can have various designs or shapes, such as oval, round, triangular, teardrop, etc.
After cutting the rough stones, you must polish their concave or curved surface for your movable streak of light to be evident.
Which gem displays chatoyancy?
Gems that show a cat’s eye effect and other optical phenomena like labradorescence or asterism are much sought after in the gem trade. Thus, they tend to be slightly more expensive than those that don’t show these phenomena. Other factors like clarity, color, cut, size, place of origin, rarity, and durability may also impact the price.
Examples of minerals or stones that display chatoyancy include the following:
1. Chatoyant chrysoberyl (BeAl2O4)
Some honey to light yellow, greenish, or dark yellowish-brown chrysoberyl gemstones display chatoyancy due to tiny parallel inclusions or tubes.
Cymophane, or cat’s eye, is a yellowish variety of chatoyant chrysoberyl that displays the clearest cat’s eye effect of all gemstones that results from microscopic rutile needle-like inclusions or tube-like cavities.
Chatoyant alexandrite is another variety of chrysoberyl that is also pleochroic and comes in various colors, including blue, red, gray, green, yellow, and purple. It is expensive, rare, and has a synthetic version known as Inamori.
2. Tiger’s eye, hawk’s eye, and chatoyant quartz
Tiger’s eye is a golden-brown chatoyant stone made of microcrystalline or cryptocrystalline quartz. This chalcedony is a metamorphic rock, and this optical phenomenon results from parallel intergrowth of quart and mainly limonite, altered crocidolite, or blue asbestos.
On the other hand, hawk’s eye (or falcon’s eye) is a bluish-gray chatoyant chalcedony that results from the intergrowth of subparallel quartz crystals and non-altered crocidolite. Another possible explanation for its formation is a pseudomorphous replacement of crocidolite is incomplete.
Besides tiger’s and hawk’s eye, translucent gray-green, yellow, brownish, greenish, or other colors may also display a cat’s eye effect. It happens due to parallel needle-like inclusions such as hornblende and rutile.
Lastly, bluish-grayish, brownish, or yellowish chatoyant pietersite is also known. This chalcedony has embedded amphibole fibers with varying levels of alteration and is what causes this optical phenomenon. Another type is amethyst, which has macrocrystalline quartz.
3. Moonstone (Na, Ca) Al₁₋₂Si₃₋₂O₈, KAlSi₃O₈)
Chatoyant moonstone cabochon may be gray, brownish-gray, blue-purple, or blue, among other colors. It occurs due to minute parallel, needle-like hematite, ilmenite, or other inclusions.
Specimens of this orthoclase variety mineral in the feldspar group may also show other optical properties like opalescence, asterism, fluorescence, and adularescence (schiller effect).
4. Sapphire and ruby
Sapphire and ruby are durable and valuable conundrum (Al2O3) varieties, some showing the cat’s eye effect and asterism. Cabochon of these minerals shows this optical phenomenon due to microscopic, parallel, needle-like rutile inclusions.
Chatoyant sapphire is mostly blue, but others with fancy colors, including yellow, purple, orange, pink, green, or violet, exist, while rubies are mainly red to blood red.
5. Chatoyant aquamarine and emerald (Be3Al2Si6O18)
Some blue, green-blue, or dark blue aquamarine, a beryl variety, shows chatoyancy due to minute parallel rods when cut in cabochon. Similarly, some bluish-green to green emerald minerals may show this optical property due to minute parallel fibrous or needle-like inclusions.
6. Chatoyant malachite
Some mineral specimens of fibrous cabochon malachite (copper carbonate hydroxide) may show a cat’s eye effect. When this greenish, often banded mineral has this optical property, its desirability and price go high.
7. Chatoyant labradorite ((Ca, Na) (Al, Si)4O8)
Labradorite is a feldspar mineral enriched with calcium, named after the place of its first identification, i.e., Labrador, Canada. Besides its spectacular labradorescence (schiller effect) optical phenomenon, some specimens of labradorites may display a cat’s eye effect.
Other chatoyant gems
These are not the only chatoyant stones. Many others include opal, tourmaline, apatite, scapolite, sillimanite, tremolite, actinolite, enstatite, and peridot. Others are nephrite, kunzite, spinel, topaz, selenite, tanzanite, ricolite, satellite, and sunstone. The list goes on.
Artificial chatoyant materials
Besides the above natural stones that show a cat’s eye effect, it is possible to find some chatoyant carbon fiber, glass such as Cathay stone, and a mineral that resembles alexandrite.
References
- Manutchehr-Danai, M. (2009). Dictionary of gems and gemology (3rd ed.). Springer.
- Webster, R., & Read, P. G. (2006). Gems: Their sources, descriptions, and identification (6th ed.). Elsevier.
- Cairncross, B., & Mccarthy, T. S. (2015). Understanding minerals & crystals (1st ed.). Struik Nature (An Imprint of Penguin Random House South Africa (Pty), Ltd.
- Read, P. G. (2005). Gemmology (3rd ed.). Elsevier Butterworth Heinemann.
How do I know a genuine and artificial chatoyant gemstone?
Some of the fake or imitated gemstones include glass like Cathay stone, carbon fiber, and alexandrite Inamori chrysoberyl. The last is sold as Victoria cat-s eye or stone.
Others are goldstone (Stellaria or aventurine glass), cat’s eye glass (cat’s eye quartz), and andara.
The key to identifying fakes and originals is understanding the properties that original gemstones have. This includes things like density, luster, appearance, etc. You may not be able to identify some of these properties on your own.
I recommend you buy your chatoyant gemstones from a trusted vendor. If you doubt, get help from a gemologist. These experts will accurately identify fake and original chatoyant gemstones.