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The Gemstone Tiger Eye
I think the gemstones, Tiger's Eye and Fire Opal. Have probably created more rock hounds, then you can shake, your proverbial stick at. It is a well known fact that Raccoons are attracted to shiny objects. Evidently most human children are too. I know my little Raccoon eyes were always drawn to the flashy color play of Opal. Just as intriguing were some giant Tiger Eye stones, owned by my mentor. They were mixed blue and gold Tiger's Eye. Which produced an ever shifting play of light.
To this day, even though I have many rare and unusual stones, in my own store. These are still two of my favorite stones. I'm not alone in this feeling either. When groups of children come to our store, more often than not, when their hands go up for questions. The questions are about, one or the other, of these two stones.
Tiger's Eye is a durable quartz composite with the usual quartz hardness of 7. It begins as the fibrous blue mineral called crocidolite, which is comprised of iron & sodium. Most of us known crocidolite as asbestos. The transformation begins when quartz becomes imbedded between the fibers of crocidolite. This process will result in one of two gemstones. A blue stone called Hawk's Eye or the golden brown stone called Tiger's Eye.
During the process, the asbestos is completely dissolved. But The quartz takes on the fibrous formations and the blue color of crocidolite. This creates the parallel lines within the gem which gives it that ever shifting play of light and movement, the stone is so loved for. This is also known as chatoyancy. This gleam that rolls across its surface. Much like the eyes of a cat.
Even though the iron & sodium dissolve, traces of hydrated oxide of iron deposit between the crocidolite and quartz, creating the golden color that is common to Tiger's Eye. How much of this hydrated mineral is deposited will determine how Golden brown, red, green or blue, Tiger's Eye and Hawk's Eye will be. The rarer blue Hawk's Eye will have only the slightest amounts.
The varying amounts of hydrated oxide of iron, actually cause several colors and mixes of color. When the color is a greenish gary, it is called cat's-eye quartz. A golden yellow reflection on a brown stone, is called Tiger's Eye. If the stone is blue gray or bluish, it's known as Hawk's Eye. Redish brown, or mahogany colored stones, are known as bull's-eye or ox-eye.
Tiger Eye is also a pseudomorph (from the Greek for "false form"). Pseudomorphs form when one mineral replaces another. Tiger-eye is a quartz replacement of crocidolite. Thus it is a pseudomorph of quartz after crocidolite. The process is similar to that which takes place when quartz replaces wood to produce petrified wood.
In my younger days the old timers maintained that Tiger's Eye was only found in Africa. Today I find this to be only a half truth. It is true that Tiger Eye's major commercial source is Cape Province, South Africa. But There are several locations that have Tiger Eye. More to the point. Tiger Eye mixed with other materials or Tiger Eye like materials. They are all quartz replacements of fibrous minerals and show some chatoyance.
Unfortunately, On May 10, 1968, Dr. Carol de Wet, then Minister of Mines of the Republic of South Africa. Announced an embargo on the exportation of uncut Tiger Eye that gradually eliminated all export of rough Tiger Eye, by May of 1971. His intent being to create a South African monopoly on finished Tiger's Eye gems. It is said that huge quantities were smuggled out. But smuggling costs money and the price of rough increased much faster than the buying public's willingness to pay. Nowadays, even at large gem shows, you really have to search long and hard to find any rough Tiger Eye. However regular golden Tiger Eye gemstones, are not too expensive. On the other hand these stones are mostly just the golden color. We have some of the older more rare color mixes.
Another rare Tiger's Eye like stone we have, is Binghamite. Binghamite is composed of quartz and fibrous silicates with inclusions of goethite and/or hematite. It is a highly prized gemmy material with a chatoyant luster, similar to Tiger's Eye. It's is usually red or yellow and may have black streaks of hemitite and sometimes blocky patches of white quartz. It can be a very interesting alternate to Tiger's Eye. It is found in the Cuyuna Iron Range of Minnesota. Silkstone is analogous to binghamite but the fibers are randomly oriented and it is not so highly prized.
Australia has a stone called Tiger Iron. Some tiger-iron is all golden just like the South African Tiger's Eye and is called Australian Tiger's Eye, by some American dealers. Some is more of a mixture, with visible streaks of red jasper. If it's mostly jasper, some dealers call it Tiger's Eye Jasper. Some also incorporates silvery gray hematite. This variety is called Tiger Iron by pretty much everyone.
Pietersite is a chatoyant, solidified crocidolite asbestos, but its appearance is quite different because it's been broken into fragments (brecciated, in mineralogical terms), stirred around, and re-cemented by silica. The result is a patchwork of shades of blue, yellow, brown, and red. Pietersite originally found in Namibia. Is now also coming from Henan Province, China. We have also seen adds for Golden Chinese Tiger's Eye. The Chinese pietersite is very similar to the Namibian one, but it has more red color and distinctive golden red combinations.
There are also some materials from California and Arizona that might qualify. The California type applies to a silica-impregnated, white-to buff-colored, massive, fibrous tremolite (another asbestos mineral) that was collected for many years at Iowa Hill in Placer County. The Arizona variety consists of fibers of chrysotile (also another asbestos mineral) in serpentine. As a rule neither of these stones are very impressive. We have heard of other localities. Such as Sri Lanka (Ceylon), India, Brazil and Burma. But I haven't seen much information on these localities.
HISTORY & FACTS:
The fibers in Tiger's Eye may be an inch or two long and very thin. Most are only 0.001 millimeters, or 0.000039 inches, in diameter.
Tiger Eye has a fibrous structure. and sawing can be tricky. Because nature seldom grows the fibers straight. They are usually bend or twisted. Cuts must be exactly parallel to the length of the fibers to get the full chatoyant effect. If the saw cut is perpendicular to the fibers. You end up with a lifeless, dark brown to black stone.
While on the subject of cutting, even though in most tiger eye the fibrous mineral has been completely replaced by quartz, recent research has shown that quartz dust can be hazardous to breathe. So when working quartz or any stone. Take adequate precautions to avoid breathing any of the dust.
In most cases Red Tiger Eye is not a natural occurrence. It is usually a result of deliberate heating. Heat treating can be done using the kitchen oven. First fill a can or some sort of metal container with sand. To protect the tiger-eye from thermal shock during heating. Cover slabs of ordinary, yellowish Tiger's Eye in fine clean silica sand, at least 3" deep. You can get this sand at your local lumber yard. It is used for sand blasting. Place them in a cold oven and increase the temperature 50°F every hour until it reached 400°F. Then turn the oven off. Do not open the door. Allow plenty of time for the container to cool all the way through.
Soon after Tiger Eye's discovery in the late 19th century. Idar-Oberstein lapidaries discovered they could bleach tiger-eye to an evenly colored light yellow. By using either hydrochloric or oxalic acid. When properly oriented and cut, this material could yield a sharp cat's-eye stone. Which was more than just a little reminiscent of "real" cat's-eye. An expensive variety of chrysoberyl.
Tiger's Eye also shows a cat's-eye effect. It just isn't the same color as the light yellow chrysoberyl. Other gems that show a similar effect are cat's-eye opal and cat's eye tourmaline. Both effects are referred to as chatoyant. "Chatoyant" comes from the French word for "cat." and these stones do gleam like a cat's eye.
Tiger's Eye is the anniversary gemstone for the 9th year of marriage.