Tiger’s eye is a golden-brown, golden-yellow, or reddish-brown semi-precious chatoyant quartz gemstone. Its other names are pseudocrocidolite, tigereye, or tiger eye.
Gemstones are said to be chatoyant if they show chatoyancy or the cat’s eye effect. Chatoyancy is an optical phenomenon in which some gemstones or minerals reflect a movable, concentrated, or sharp, silky band of light when cut en cabochon.
This luminous, concentrated band of light resembles the pupils of the eyes of cats, which appear as vertical slits. The resemblance of this optical phenomenon with the eyes of a cat is the reason it is called cat’s eye effect, or chatoyancy. Chatoyancy comes from a French word ‘chat oeil’ that means the eyes of a cat.
Chatoyancy in minerals and gemstones is due to their fibrous structure or the presence of tiny, nearly parallel fiber, needle-like channels, tubes, cavities, or other inclusions. Some of these cavities, channels, or tubes are empty, fluid-filled, or may contain other minerals.
We have a separate article on chatoyancy in gemstones and minerals, which elaborates more on the causes. It will also give you the 10 common chatoyant minerals, including quartz, opal, alexandrite, moonstone, chrysoberyl, sapphire, rubies, aquamarine, and tourmaline, amongst others.
What causes tiger’s eye?
Tiger’s eye occurs due to an intergrowth of nearly parallel quartz crystals and altered crocidolite or blue asbestos fiber, usually limonite. The crocidolite fiber inclusions reflect light perpendicular to their surface. This reflected light is what causes the narrow, concentrated, silky luminous streak or band of light.
Crocidolite or blue asbestos (Na2Fe3Fe2[(OH)Si4O11]2) is a sodium-rich riebeckite. Riebeckite is a blue-gray, indigo-blue, or leek-green sodic amphibole that has needle-like fibers with parallel sides.
The partial alteration causes blue asbestos to turn golden-yellow, golden-brown, or reddish brown.
This chatoyant gemstone is called the tiger’s eye because it looks like the eyes of a tiger. It is golden-yellow, golden-brown, or reddish-brown and reflects a concentrated, silky band of light.
The eyes of a tiger are yellowish, golden, yellow, or yellowish orange and have vertical pupils that appear as a movable, darker streak.
Properties of tiger’s eye
Tiger’s eye is translucent or nearly opaque, a golden-yellow, reddish brown, or golden yellow chatoyant gemstone. Some specimens can be honey, bronze, yellowish, or banded, including blackish bands.
This gemstone has a brittle tenacity, a silky luster, a Mohs hardness scale of 7, a density of 2.64–2.71 g/cm3, and a conchoidal fracture.
Finally, its chemical composition is mostly quartz (silicon dioxide) and a small amount of limonite inclusions.
Formation
Tiger’s eye forms either by partial replacement of altered crocidolite by quartz or crack-seal vein-filling process.
1. Replacement of altered crocidolite by quartz
The replacement of altered crocidolite is a pseudomorphism process, meaning replacement happens while retaining the original mineral structure. During this silicification process, quartz replaces the wavy, nearly parallel altered fibrous crocidolite but retains the fibrous asbestiform structure.
The process starts with the alteration of blue crocidolite to golden-yellow, yellow-brown, or reddish brown. The alteration forms limonite or iron III oxide responsible for the tiger’s eye coloration, and it involves the conversion of Fe2+ to F3+.
After the alteration processes, groundwater containing dissolved silica will seep into and replace some of the crocidolite fibers. This replacement process retains the fibrous structure of asbestos and some of the hydrous iron oxide, forming silicified crocidolite.
The formed, nearly parallel quartz fibers with some limonite inclusions reflect light perpendicular to the surface, causing chatoyancy.
2. Crack-seal vein filling process
The crack-seal vein filling process forms tiger’s eye when water containing quartz and crocidolite seeps into microcracks that occur in quartz-rich veins found in the upper and middle crust.
Microcracks will fill as new ones form and this repeated process creates the hawk’s eye which are blue-grayish in color.
The existing microcracks will fill; new ones will form and fill again. This process occurs repeatedly, forming the bluish-gray hawk’s eye gemstone. However, the tiger’s eye gemstones will only form if weathering of crocidolite occurs.
Crocidolite weathering creates iron oxides and hydroxide coats that will change the blue asbestos color to yellow-brown or reddish-brown.
Peter Heaney, a mineralogist at Pennsylvania State University, and his co-author Don Fisher proposed crack-seal and vein filling theory in their 2003 paper.
This conclusion was after a microscopic study of a tiger’s eye gemstone from South Africa, which showed no fibrous crocidolite inclusions, tiny fibrous quartz, or chalcedony.
Heaney and Fisher noted that it was unlikely that a tiger’s or hawk’s eye formed from the pseudomorphic replacement process of altered crocidolite.
Dave V. Wiltschko, a Texas A&M University in College Station structural geologist, however, seems to disagree with the crack-seal vein filling theory. He points out that there would be tiny fluid pockets and evidence of deformation on quartz in the crust.
What is tiger’s eye used for?
Tiger’s eye is an inexpensive gemstone that makes cabochons, curved cameos, hearts, pendulums, spheres, ashtrays, engraved gemstones, plugs, pyramids, and other ornaments in various shapes.
This chatoyant gemstone also makes beads, earrings, anklets, bracelets, curved rings or bands, ring centerpieces, brooches, studs, and pendants.
Where is tiger’s eye found?
Tiger’s eye gemstones are common in the Northern Cape in South Africa, especially at Griqualand, Prieska, Niekerkshoop, Hay, Balloch Mine, and Griekwastad. It is also found in Australia (at Mount Brockman), Canada, United States, India, Namibia, China, Myanmar, Brazil, Madagascar, Bolivia, and Burma.
Places in the US with these gemstones include California, Arizona, Montana, and Georgia (Lilly Pad Village). Tiger’s eye in Arizona and California, however, has serpentine with fibrous chrysotile inclusions.
Toxicity
Blue asbestos is carcinogenic (cancer-causing) and is associated with mesothelioma and asbestosis. This raises concerns about the safety of tiger’s eye gemstones. Are they harmful, toxic, or dangerous too?
The answer is no. Tiger’s eye only has a tiny amount of unliberated blue asbestos or crocidolite locked deep inside quartz. This makes these gemstones safe. You can wear, wash, or use them as you please.
Only free, petrified, or powdered asbestos is dangerous to your health.
Heat treating this gemstone
Heating the usual golden-brown, golden-yellow tiger’s eye at a temperature of 400°C (752°F) will turn its color to reddish-brown. Heating converts limonite to hematite but doesn’t affect chatoyancy.
How do you spot a fake tiger’s eye?
We don’t know of any fake tiger’s eye gemstones on the market apart from those that are dyed, or heat treated to enhance their color. Furthermore, if you consider properties like their density, fracture, luster, and diaphaneity, it is easy to spot faux ones.
It should scratch a steel knife or glass, have a silky luster, and be translucent or almost opaque. The density of this gemstone should be 2.64–2.71 g/cm3, and it should break with a conchoidal fracture.
Frequently asked questions?
Hawk’s or falcon’s eye is a blue-gray chatoyant gemstone in which the crocidolite is unaltered.
The zebra tiger’s eye, or zebra crocidolite, is a multi-colored variety with golden brown and blue-gray bands. The areas with golden brown bands have altered crocidolite, while the blue gray sections have unaltered crocidolite.
The grayish-green cat’s eye is a tiger’s eye with actinolite inclusions. Actinolite is a sodium-rich amphibole belonging to the asbestos group, and it is what gives this chatoyant gemstone the greenish hue.
Griqualandite is a chatoyant yellow-brownish or reddish brown silicified blue asbestos or crocidolite with silica and hydrous iron oxide. This name comes from Griqualand, formerly Cape Colony in South Africa, where silicified crocidolite was mined at the Asbestos Mountains.
Pietersite is a fractured or brecciated tiger’s eye gemstone from China and Namibia. It is blue-gray, yellow-brown, or brownish-red in color and has crocidolite fibers, which cause chatoyancy.
These are tiger’s eye gemstones, which are dyed with inorganic pigments to enhance their appearance and give them other colors. They come in various colors like pink, green, blue, fuchsia, magenta, dark blue, teal, golden blue, purple, teal, and rainbow. Dying is a common practice, and it doesn’t alter the original properties of these gemstones.
It is a metamorphic rock with mainly quartz, a silica polymorph, and a tiny amount of blue asbestos. It is not a crystal or mineral.
No. Tiger’s eye is not a magnetic gemstone. It has quartz and blue asbestos, both of which are non-magnetic minerals. However, specimens like Tiger iron, which have black hematite or red jasper, are slightly magnetic.
Prices range from less than a dollar per carat, with the most popular kinds going for up to $3 per carat. Factors that will affect price include appearance, size, origin, cut, and so on. Uncut or raw, cut and polished, or tumbled gemstones will have different prices.