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Specific PropertiesThe most obvious property and the reason for its use as a gemstone is the silky shine that can be seen in polished, and especially in tumbled and cabochon-cut stones. All "eyes" show a wavy pattern of parallel lines that seem to move when the stone is turned. Smaller specimen will show only one or two long lines. This effect is named chatoyance because its look resembles an eye of a cat ("chat" is French for cat). It can also be seen in other minerals, for example in chrysoberyl, which is then named "oriental cat's eye".
Hawk's eye is a partially silicified fibrous blue-gray crocidolite asbestos, so its fibrous structure is due to embedded fibers of crocidolite in quartz. It is usually very dark and almost opaque.
In tiger's eye the crocidolite fibers appear to have been fully replaced by quartz and the hydrous iron oxide goethite, FeOOH, the latter being responsible for the yellow-brown color. Though brighter than hawk's eye, tiger's eye is still almost opaque. The former fibrous structure is preserved in the orientation of the small columnar quartz crystals that grew perpendicular to the walls of the vein. Due to its fibrous structure, tiger's eye has a somewhat splintery fracture, not unlike wood.
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Cat's eye is typically more translucent and brighter, and occurs in a variety of colors, green, yellow, brown, and rarely blue. The cause of its chatoyance can be embedded fibers of asbestos, as well as actinolite fibers.
Asbestos is a well known useful but hazardous material and can cause lung cancer, but its dangerous fibers are well embedded in hawk's eye and cat's eye and pose no danger for its owner. They are completely replaced by quartz and iron oxides in tiger's eye.
According to the more common theory, tiger's eye would present a pseudomorph of quartz after crocidolite. Occasionally hawk's eye shows areas of yellow color, corresponding to tiger's eye, and this is interpreted as an indication that hawk's eye represents an intermediate step in the alteration of crocidolite to quartz and iron oxides. This assumption is further supported by the fact that both tiger's eye and hawk's eye occur in the same veins as crocidolite, however in general only at places where these veins appear at the surface. This top layer is said to have been silicified starting from the surface. If one follows a vein into the ground, one will first find tiger's eye, then at most places after a few meters hawk's eye, and finally crocidolite asbestos. So the silicification at most places extends further into the ground than the oxidative alterations, and both processes are considered secondary - they apparently occurred after the crocidolite veins have formed, the silicification only for some period in time, while the oxidation is an ongoing process.
However, recently objections have been raised about this theory by Heaney and Fisher (2003). These objections are mainly based on mineralogical, in particular crystallographic observations on tiger's eye and hawk's eye specimen that support a syngenetical (simultaneous) formation and possibly epitactic intergrowth of crocidolite fibers and quartz. After I have looked at thin sections of tiger's eye under a polarizing microscope, I find the simultaneous formation of quartz and crocidolite fibers likely. Tiger's eye is made of elongated quartz crystals that, like the crocidolite fibers, grew roughly perpendicular to the fissure walls. The altered crocidolite is embedded in the quartz crystals as very thin fibers that contribute only a small percentage of the overall mass. There is no sign of dissolution of the fibers, and the quartz crystals are perfectly clear and do not show casts of metasomatically replaced fibers. I wouldn't call the intergrowth of quartz and crocidolite fibers epitactic though, because the fibers do not strictly follow the crystallographical directions of the quartz crystals and occasionally bend inside the crystals.
OccurrenceTiger's Eye and Hawk's Eye occur in cracks of rock that once have been filled by asbestos fibers. The cracks and clefts are rarely more than a few centimeters wide, but may extend over several meters, resulting in platy specimen.
Locations and SpecimenMost tiger's eye and hawk's eye comes from the Asbestos Mountains near Griquatown, South Africa.
Cat's eye is found in India and Sri Lanka.
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