Falcon, hawk’s cat’s eye, or blue tiger’s eye refers to a blue to bluish-grayish, semi-precious, inexpensive, chatoyant quartz gemstone formed by the intergrowth of subparallel quartz and crocidolite fibers or blue asbestos.
This translucent to nearly opaque gemstone is a quartz metamorphic rock, i.e., it is made of silica or silicon dioxide (SiO2) mineral with some crocidolite inclusion that makes it chatoyant. Its uses include making jewelry (rings, bracelets, pendants, earrings, etc., and some decorative ornaments.
Discover more about the hawk’s eye, including how it forms, its properties, prices, uses, and much more. However, since it is not our purview, we will not discuss this gemstone’s metaphysical, astrological, spiritual, or healing properties or significance.
What a hawk’s eye stone is
Chatoyancy is an optical phenomenon in some gemstones when correctly cut en cabochon. It describes the reflection of a concentrated, narrow, bright band of light with a silky sheen that seems to move across the curved dome-like surface as you turn it. Falcon’s or hawk’s eye is only a type.
The single movable streak of light resembles the vertical slit seen in a cat’s eye. Thus, this phenomenon is also known as cat’s eye effects. It occurs when microscopic parallel fibers, tubes or channels, or needle-like inclusion reflect light perpendicular to their orientation.
Minerals that show this optical phenomenon are said to be chatoyant or named with the words cat’s eye. For instance, chrysoberyl, opal, quartz, or aquamarine cat’s eye imply they show chatoyancy.
In the case of the falcon’s eye, chatoyancy results from light reflection by minute crocidolite fibers when this gemstone is appropriately cut en cabochon. However, there is a contrary school of thought. We will discuss more when looking at formation.
The falcon’s or hawk’s eye name of this chatoyant quartz is because it has a bluish-gray coloration that resembles some falcons and or hawks. Sometimes, people call it the blue tiger’s eye to emphasize the bluish color that comes from the inclusion of blue asbestos. However, a tiger’s eye stone has golden yellow, honey, golden brown to reddish brown chatoyant quartz with altered crocidolite fibers.
Properties
Falcon’s eye properties include the following:
- Chemical composition: Silicon dioxide (SiO2) or silica with blue asbestos ((Na2Fe3Fe2[(OH)Si4O11]2 inclusions, i.e., a sodic amphibole group mineral
- Colors: Blue, dark blue, navy blue to bluish gray but may have golden-yellow to golden brown patches or streaks. Some may be greenish blue but are rare.
- Mohs hardness: 6.5-7 Mohs scale
- Luster: Silky
- Fracture: Conchoidal
- Specific gravity: 2.64 – 2.71
- Diaphaneity: slightly translucent to nearly opaque
- Tenacity: Brittle
- Refractive index: 1.534 to 1.540
How is a hawk’s eye formed?
Two theories explain how these gemstone forms. One is via partial replacement of crocidolite by quartz (silica), and the other is the crack and seal vein filling process.
1. Partial replacement of crocidolite
The oldest explanation for the formation of the hawk’s eye is via partial pseudomorphous replacement of crocidolite or blue asbestos (asbestiform riebeckite) by quartz. During this partial silicification process, remnant subparallel crocidolite fiber becomes embedded in nearly colorless quartz.
The remaining embedded crocidolite fibers make this gemstone chatoyant or display the cat’s eye effect, giving it a unique blue, dark blue, bluish-gray, and rarely greenish-blue color.
If the bluish-gray crocidolite alternates to limonite before pseudomorphous replacement, you will end up with golden-yellow, golden-brown honey to reddish-brown tiger’s eye. The coloration is due to the staining of quartz by iron (III) oxide formed due to alteration.
2. Crack-seal vein-filling process
Peter Heaney and Don Fisher proposed concurrent growth of quartz and asbestos via crack-seal vein filling to be how tiger’s or unaltered hawk’s eyes formed in 2003.
This theory was after a microscopic examination showed a South African tiger’s eye sample didn’t have fibrous quartz or chalcedony. It means that quartz’s pseudomorphic replacement of blue asbestos while retaining the fibrous, subparallel structure didn’t occur.
According to their paper, quartz-rich vein develops micro-fractures, which are then water containing quartz and crocidolite enter the microcracks. Quartz starts growing on the surface of the microcracks surrounding the growing crocidolite fibers on the crack surface. This process repeats numerous times for a long time, forming the hawk’s eye stones.
Hawk’s eye stone uses
Uses of hawk’s eye gemstones include making jewelry like bracelets, rings, and pendants, especially using cabochons, beads, hearts, or faceted pieces. Also, they are small ornamental objects like pyramids, plugs, spheres, prisms, or mushrooms and engraved or miniature carvings (scarabs, intaglios, and cameos) or even tumbled chips.
Dyed falcon’s eye
Natural hawk’s eye gemstones come in bluish-gray hues, including blue, dark blue, and rarely blue-green. Treatment with nitric acid can help lighten colors. Also, some people may dye these gemstones to get other colors, and most vendors will indicate it. However, you can still confirm by asking.
Imitations
Unless dyed or enhanced, most hawk’s eye gemstones are natural, i.e., no known synthetic ones or fakes. However, chatoyant fiber optic glass is a common imitation in cabochons, spheres, triangles, cubes, stars, hearts, and other shapes available in almost every color.
FAQs
Falcon’s or hawk’s eye gemstones have crocidolite or blue asbestos, which are the most harmful and cause asbestosis and mesothelioma. However, since it is unliberated, i.e., embedded in quartz, it is harmless. Thus, hawk’s falcon eye’s gemstones are not toxic or dangerous. You can even wash or even wear them every day.
Major deposits in Griqualand and other parts of Northern Cape province in South Africa, where it is known as blue tiger’s eye, and some parts of Western Australia, especially Mt. Brockman and Hamersley range. Also, it occurs in Brazil, Canada, China, India, Sri Lanka, Myanmar, Namibia, and the United States.
The price of hawk’s or falcon eye gemstones ranges from about 10 cents to about $3 per carat. Prices depend on factors such as place of origin, colors, clarity, size, cut, and rarity. Raw, rough, uncut hawk’s eye stones cost less than cut and polished pieces.
Yes. Blue tiger’s eye (falcon’s or hawk’s) is a naturally occurring bluish to bluish-gray semi-precious gemstone. This stone is quartz metamorphic rock made of quartz (silica or silicon dioxide) with some embedded crocidolite or blue asbestos inclusions that give it the characteristic color.
References
- Manutchehr-Danai, M. (2009). Dictionary of gems and gemology (3rd ed.). Springer.
- Okrusch, M., & Frimmel, H. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits. Springer. https://doi.org/10.1007/978-3-662-57316-7
- R. V. Dietrich © 2015. Tiger’s-eye & hawk’s-eye. Central Michigan University. http://stoneplus.cst.cmich.edu/hawkseye.htm
- Price, M., & Walsh, K. W. (2005). Rocks and minerals (1st ed.). Dorling Kindersley.
- Heaney, P. J., & Fisher, D. M. (2004). New interpretation of the origin of Tiger’s-Eye: Comment and reply. Geology, 32(1). https://doi.org/10.1130/0091-7613(2003)031%3C0323:NIOTOO%3E2.0.CO;2
Hey there! I just read this interesting article about the Falcon or Hawk’s Eye stone, also known as Blue Tiger’s Eye – I wonder if anyone has ever spotted a real blue tiger with these incredible eyes?