Chatoyancy (chatoyance) or cat’s eye effect in gemology is an optical phenomenon where some gemstones, minerals, or rocks reflect a movable, sharp, or concentrated silky band of light when cut en cabochon.
This mystical, wavy, or silky luminous band of light or sheen emanates from beneath the surfaces of these gemstones, minerals, or rocks. The concentrated streak of light also moves across the curved surface of a polished cabochon when you move the light source or turn the cabochon.
Gemstones or minerals that show this optical phenomenon are said to be chatoyant. Tiger’s eye, hawk’s, or falcon’s eye are variations of chatoyant gemstones and minerals.
The term chatoyancy is of French origin. It comes from the word ‘chat oeil’ or ‘oeil de chat’ which means the eyes of a cat. It refers to the slit-like shine or shimmer that some gemstones, minerals, and rocks reflect, which resembles that of a cat’s eye.
Some sources note that the name chatoyancy comes from the French word ‘chatoyer’, which means to shine or shimmer like the eyes of a cat.
To visualize this optical effect, just look into a cat’s eyes. You see a vertical, slit-like gleaming iris that appears to move as you move the light source, just as chatoyant stones do.
Besides minerals or gemstones, a silk or nylon thread reel also reflects a single, concentrated, or narrow band of light.
Common minerals that display chatoyance are quartz, aquamarine, ruby, apatite, tourmaline, sapphire, and moonstone. Others are chrysoberyl, alexandrite, malachite, and labradorite.
Some wood and artificial materials like carbon fiber, glass, and Cathay stone can also display chatoyancy.
What cause chatoyancy in minerals and gemstones?
The cat’s eye effect, or chatoyancy, is caused by a fibrous structure or the presence of numerous elongated, parallel fibers, channels, cavities, tubes, and other inclusions in gemstones. The tubes or cavities may be empty, fluid-filled, or contain other minerals.
These fibrous structures, parallel needle-like cavities or inclusions, reflect light perpendicular to their surface. This reflected light is what causes chatoyancy, or a luminous band or streak of light.
Turning the gemstone or the angle of incident light makes the luminous streak appear as though it moves on the curved surface of a cabochon.
For gemstones or minerals to display chatoyancy, the inclusions must have the same orientation, i.e., direction and plane within the host crystal lattices. Those with different orientations will cause asterism, another optical phenomenon.
Factors that affect the intensity of the reflected luminous streak of light are the density and thickness of gemstones or minerals, as well as the orientation of fibers and other inclusions. It is, therefore, possible to have gems with varying degrees of chatoyancy.
Examples of chatoyant gemstones
Common gemstones or minerals that display chatoyancy are:
1. Chrysoberyl (BeAl2O4)
Chrysoberyl is one of the most common gemstones that displays chatoyancy due to the presence of numerous parallel fiber-like rutile or tube-like cavity inclusions.
The popular chatoyant chrysoberyl gemstones are cymophane and alexandrite. Cymophane, also known as oriental cat’s eye or cat’s eye, is a yellowish variety that displays the clearest chatoyancy of all gemstones.
Alexandrite, on the other hand, is a rare and expensive variety that comes in yellow, blue, red, green, purple, gray, or other colors. This gemstone is also pleochroic.
Depending on their quality, size, place of origin, cut, and rarity, these chatoyant gemstones cost $15 for the yellow-green, $60 for the brownish, while the clearest honey can fetch as much as $2,000 per carat.
2. Quartz (SiO2)
Chatoyant quartz has parallel fibers, channels, or needle-like inclusions of hornblende, rutile, asbestos, or actinolite. This translucent gemstone comes in gray green, greenish, yellow, brownish, or other colors.
Tiger’s eye is one of the popular golden-brown chatoyant quartz varieties with altered fibrous crocidolite and quartz crystals. The other common variety is blue-gray hawk’s or falcon’s eye with intergrowth of quartz and blue asbestos or crocidolite fibers.
Besides tiger’s and hawk’s eye, the other chatoyant quartz varieties are pietersite and amethysts. Pieterstite is blue-gray, brownish, or yellowish, while amethyst is a microcrystalline variety of quartz.
3. Moonstone Na, Ca) Al₁₋₂Si₃₋₂O₈, KAlSi₃O₈
A chatoyant moonstone has tiny, nearly parallel, or fibrous hematite, ilmenite, and other inclusions. Moonstone is a variety of orthoclase that is usually gray, blue, blue-purple, or brownish gray, but it can be other colors.
Besides chatoyancy, some moonstone specimens show other optical phenomena like asterism, fluorescence, opalescence, or the Schiller effect (adularescence). The milky-white shimmery moonstone is, however, not chatoyant.
4. Opal (SiO2·nH2O)
Some of the brown, yellow, or green varieties of opal, a hydrated silica mineraloid, show chatoyancy, caused by minute hollow tubes or fibrous golden, white, or brown inclusions.
Another chatoyant variety is harlequin opal, which can be flashfire, green, or multicolored.
5. Sapphire and ruby conundrums (Al2O3)
Chatoyant rubies and sapphire have microscopic parallel, needle-like inclusions. Sapphires are often blue but can be yellow, pink, purple, green, violet, or orange, while rubies are mostly red or blood red.
Chatoyant rubies and sapphires are conundrums with a Mohs hardness scale of 9. Their uses include making rings, necklaces, pendants, earrings, or bracelets, and their prices range from 50s to 100s of dollars per carat.
6. Tourmaline
Chatoyant tourmaline has fibrous crystals or hollow needle-like cavities that are filled with fluid, gas, or other minerals. This gemstone is usually green, red, or pink but can have other colors like brown, yellow, colorless, blue, blue-green, or yellow green.
7. Aquamarine and emerald (Be3Al2Si6O18)
Aquamarine and emerald are popular beryl varieties that sometimes display the cat’s eye effect.
Chatoyant aquamarine gemstones are usually blue, green-blue, or dark blue and have microscopic rod inclusions.
Chatoyant emeralds, on the other hand, are blue green to greenish in color and have fibrous or parallel elongated inclusions.
Some of the uses of these gems include making earrings, necklace pendants, earrings, and brooches.
8. Scapolite
Chatoyant scapolite has parallel needle-like inclusions, channels, rods, or cavities. Some of the cavities are fluid filled, others hollow or with minerals like pyrrhotite or reddish-brown iron oxides. These gemstones are white, pale yellow, blue-violet, pink, or black.
9. Apatite
Some of the translucent, fibrous apatite varieties may show or display the cat’s eye effect. Chatoyant apatite gemstones are often green, yellow-green, blue, brown, or reddish brown.
10. Synthetic chatoyant materials
Beside the natural chatoyant gemstones, synthetic ones exist. Synthetic chatoyant materials include Victoria cat’s eye or stone (alexandrite Inamori chrysoberyl), carbon fiber, and glass varieties like Cathay cat’s eye or Cathay stone.
How do you cut chatoyant gemstones
To see the cat’s eye effect, cut your gemstones or minerals en cabochon. Cabochons have a convex top with no faucets and a flat bottom. You can have various shapes of cabochons, including triangular, round, or teardrop.
When cutting these stones, ensure the base is parallel to the plane or orientation of the inclusions or fibers. The fibers or inclusions should also run symmetrically to your cabochon and preferably along its length.
Once you have cut your cabochon correctly, polish the curved surface for the best chatoyancy display.
How much are chatoyant gemstones?
Chatoyant or other gemstones that show special optical phenomena like asterism and labradorescence are in high demand in the gemstone trade. Their prices vary from a few to thousands of dollars depending on the specific mineral, clarity, color, place of origin, quality, size, and other factors.