In
gemology
Gemology or gemmology is the science dealing with natural and artificial gemstone materials. It is a geoscience and a branch of mineralogy. Some jewelers (and many non-jewelers) are academically trained gemologists and are qualified to identify a ...
, chatoyancy ( ), or chatoyance or cat's eye effect, is an
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
reflectance effect seen in certain
gemstone
A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, ...
s, woods, and
carbon fibre
Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
. Coined from the French "œil de chat", meaning "cat's eye", chatoyancy arises either from the fibrous structure of a material, as in
tiger's eye
Tiger's eye (also called tiger eye) is a
chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre. As members of the quartz group, tiger's eye and the related blue-coloured mineral hawk's eye g ...
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
, or from fibrous inclusions or cavities within the stone, as in cat's eye
chrysoberyl
The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula Be Al2 O4. The name chrysoberyl is derived from the Greek words χρυσός ''chrysos'' and βήρυλλος ''beryllos'', meaning "a gold-white spar". Despite ...
.
Description
The precipitates that cause chatoyance in
chrysoberyl
The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula Be Al2 O4. The name chrysoberyl is derived from the Greek words χρυσός ''chrysos'' and βήρυλλος ''beryllos'', meaning "a gold-white spar". Despite ...
are the mineral
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer Polymorphism (materials science), polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest ...
, composed mostly of
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolubl ...
. Examined samples have yielded no evidence of tubes or fibres. The rutile precipitates all align perpendicularly with respect to cat's eye effect. It is reasoned that the lattice parameter of the rutile matches only one of the three orthorhombic crystal axes of the chrysoberyl, resulting in preferred alignment along that direction.
![Cat's Eye super cropped III](https://upload.wikimedia.org/wikipedia/commons/c/c6/Cat%27s_Eye_super_cropped_III.jpg)
The effect can be likened to the sheen off a spool of silk: The luminous streak of reflected light is always perpendicular to the direction of the fibres. For a gemstone to show this effect best it must be cut
en cabochon
A cabochon (; ) is a gemstone that has been shaped and polished, as opposed to faceted. The resulting form is usually a convex (rounded) obverse with a flat reverse. Cabochon was the default method of preparing gemstones before gemstone cuttin ...
(rounded with a flat base rather than faceted), with the fibres or fibrous structures parallel to the base of the finished gem. The best finished specimens show a single sharply defined band of light that moves across the stone when it is rotated. Chatoyant stones of lesser quality display a banded effect as is typical with cat's-eye varieties of quartz. Faceted stones do not show the effect well.
Gem species known for this phenomenon include the aforementioned
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
,
chrysoberyl
The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula Be Al2 O4. The name chrysoberyl is derived from the Greek words χρυσός ''chrysos'' and βήρυλλος ''beryllos'', meaning "a gold-white spar". Despite ...
,
beryl
Beryl ( ) is a mineral composed of beryllium aluminium silicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include emerald and aquamarine. Naturally occurring, hexagonal crystals of beryl can be up to several mete ...
(especially var.
aquamarine),
charoite
Charoite is a rare silicate mineral, first described in 1978. It was believed to be named after the Chara River, but due to the river being 70 km away from its discovery place, now it is believed to be named after the Russian word ''chary'', ...
,
tourmaline
Tourmaline ( ) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is a gemstone and can be found in a wide variety of colors.
The ...
,
labradorite
Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect ( schiller).
Labradorite is an intermediate to calcic member of the plagioclase series. It ...
,
selenite
Selenite may refer to:
Substances containing selenium
*A selenium-containing anion or ionic compound with the SeO32− anion:
**Selenite (ion), anion is a selenium oxoanion with the chemical formula SeO32−
***Selenous acid, the conjugate acid, w ...
,
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
,
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...
,
moonstone,
thomsonite
Thomsonite is the name of a series of tecto-silicate minerals of the zeolite group. Prior to 1997, thomsonite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a seri ...
and
scapolite
The scapolites (Gr. σκάπος, rod, and λίθος, stone) are a group of rock-forming silicate minerals composed of aluminium, calcium, and sodium silicate with chlorine, carbonate and sulfate. The two endmembers are meionite () and marialit ...
amongst others. Glass optical cable can also display chatoyancy if properly cut, and has become a popular decorative material in a variety of vivid colors.
The term "cat's eye", when used by itself as the name of a gemstone, refers to a cat's eye chrysoberyl. It is also used as an adjective which indicates the chatoyance phenomenon in another stone, e.g., cat's eye
aquamarine.
In woodworking
Chatoyancy in wood occurs in various species – particularly
hardwood
Hardwood is wood from dicot trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood (which comes from ...
s and the various types of
Nanmu
Nanmu () is a precious wood that is unique to China and South Asia, and was historically used for boat building, architectural woodworking, furniture and sculptural carving in China. The Ming Dynasty-era writings indicate this wood as superior dura ...
woods of China and South East Asia, particularly where stresses from the weight of the growing tree result in denser patches, or where stresses cause
burl
A burl (American English) or burr (British English) is a tree growth in which the grain has grown in a deformed manner. It is commonly found in the form of a rounded outgrowth on a tree trunk or branch that is filled with small knots from do ...
or ''
bird’s eye''. This ‘figure’, which has a striking three-dimensional appearance, is highly prized by woodworkers and their clients alike, and is featured regularly in furniture, musical instruments, and other decorative wood products. Figuring takes on a variety of forms and is referred to as ''flame'', ''ribbon'', ''tiger stripe'', ''quilting'', among other names.
This effect is sometimes called ''wet look'', since wetting wood with water often displays the chatoyancy, albeit only until the wood dries. Certain finishes cause the
wood grain
Wood grain is the longitudinal arrangement of wood fibers or the pattern resulting from such an arrangement.
Definition and meanings
R. Bruce Hoadley wrote that ''grain'' is a "confusingly versatile term" with numerous different uses, including ...
to become more pronounced. Oil finishes,
epoxy
Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also coll ...
, and
shellac
Shellac () is a resin secreted by the female lac bug on trees in the forests of India and Thailand. It is processed and sold as dry flakes and dissolved in alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and ...
can strongly bring out the ''wet look'' effect. When the refractive index of the finish nearly matches that of the wood, light scattering no longer occurs at the wood surface, adding the appearance of depth to the wood's figure.
Measurement
No method to measure wood chatoyance is unanimously accepted by the scientific community. Some methods have been proposed, such as one named PZC, which was used to measure typical values for a number of wood species; some results are reported below:
See also
*
*
References
;General
* Webster, R., Jobbins, E. A. (Ed.). (1998). ''Gemmologist's compendium.'' St Edmundsbury Press Ltd, Bury St Edwards.
*Mitchell, T. et al. ''Proceedings of the Electron Microscopy Society of America (EMSA)'', 1982.
{{Refend
Mineralogy
Optical phenomena