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FEATURE STORY
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20 Questions: some that are frequently asked, and others we just think are interesting. By Si & Ann Frazier, Foreign Correspondents |
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Yes, at least five that are recognized by most gem authorities. In approximate order of our perception of their commercial importance, they are: hawk's-eye, tiger-iron, pietersite, binghamite, and silkstone. Like tigereye, they are all quartz replacements of fibrous minerals and show some chatoyance. These are described in more detail in Questions 15-18. |
The good news is that tigereye is quite abundant at its major commercial source in Cape Province, South Africa, where the extensive deposits are right on or near the surface and easily worked, making tigereye not only abundant but also inexpensive. More good news is that finished stones are also plentiful on the market. The bad news is that rough is not. |
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 tigereye that gradually eliminated all rough export by May of 1971, the intent being to create a South African monopoly on finished tigereye gems. Although this meant risking the ire of important trading partners, the government - then a pariah among nations, anyway, because of its policy of Apartheid - must have figured it had little to lose in the public relations arena. Not surprisingly, however, the measure, still in effect today, worked about as well as most government prohibitions: huge quantities were smuggled out instead of legally exported. We know of one large warehouse in a northern German port city that is stuffed with tons of rough tigereye, and large-volume commercial cutters in the German cutting center of Idar-Oberstein and in the Far East have no trouble obtaining all the rough they want. Smuggling costs money, though, and the price of rough increased much faster than the buying public's willingness to pay a premium for finished gems, stifling any inclination by manufacturers to promote the stone. The biggest effects of the ban were actually to reduce the overall popularity of tigereye, which had been a best-seller among cabochon-cut gems prior to the embargo, and to vastly reduce the supply of rough available to small-volume cutters. Nowadays, even at the huge Tucson or Denver gem shows, you really have to search long and hard to find any rough tigereye. Even so, at less than a dollar a cab for average sizes and four to five dollars apiece for larger stones, tigereye packs a lot of visual pizazz for the buck. |
Neither. It's actually a granular quartz with the c axis (usually the length of a quartz crystal) at right angles to the direction of the former asbestos fibers (see Question 1). Chalcedony, on the other hand, consists of tightly compacted, very tiny (microscopic), fiberlike aggregates of quartz. Even though chalcedony has a slightly lower hardness (Mohs: 61/2), its structure makes it tougher than a crystalline quartz (7 in hardness) such as rock crystal or amethyst. Agate, by the way, is a type of chalcedony: one with a banded or other attractive pattern. Of the crystalline quartz gems, those that are finely granular, such as tigereye, hawk's-eye, aventurine, and dumortierite quartz, are generally quite tough and durable, making them suitable for jewelry that might be subjected to hard usage, such as a ring. |
Not too many people ask this question, but we know the answer! The fibers may be an inch or two long and are really, really skinny: most are only 0.001 millimeters, or 0.000039 inches, in diameter! (Bauer and Schlossmacher, 1931, 673.) |
Polish. |
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In the near-surface weathering conditions in the arid climate of the Northern Cape Province of South Africa where tigereye is mined, the crocidolite asbestos veins slowly become impregnated with silica. When enough silica has accumulated to solidify the material sufficiently to make it a quartz lapidary product, it is the deep blue of the original crocidolite fibers. This material is called hawk's-eye (see Question 15). The crocidolite is a sodium iron silicate. As oxidation proceeds, the crocidolite fibers start to break down, with the iron silicate turning into iron oxide-hydroxide in the form of the mineral limonite, which is basically rust and corresponds in composition to the yellow ochre used by jewelers as a heat sink. The yellow-brown of the limonite mixed with the original blue gives a greenish color. Natural green tigereye is, in our experience, very attractive but not at all abundant. As oxidation proceeds further, the crocidolite breaks down further, and the tigereye becomes the yellow-brown that is most familiar. Heating (which may occasionally occur naturally) can turn the yellow limonite into red hematite, making red tigereye. |
Don't be so naive. When tigereye is heated, the yellow limonite that imparts its color turns to red hematite, and the tigereye turns red, too (see Question 10). Wildfires can provide enough heat to redden a number of quartz materials, but the area where the tigereye comes from isn't heavily enough vegetated to support many wildfires. However, tigereye miners deliberately build fires to break apart the hard rock enclosing the seams of tigereye, and that heat, too, can turn the tigereye red. Back in the 1960s, we were able to buy (at a considerable premium) occasional sacks of very attractive tigereye with splashes of yellow, peach, and red, presumably the result of the miners' use of fire to break the rock. It's easy to make red tigereye on purpose. We've done it overnight using the kitchen oven. First we filled some coffee cans with sand to protect the tigereye from thermal shock during heating. Then we put slabs of ordinary, yellowish tigereye in the cans and placed them in a cold oven, which we heated by increasing the temperature 50°F every hour until it reached 400°F. Then we turned the oven off and allowed it to cool overnight. When we took the slabs out the next morning, they were a nice shade of red. There are many variations on this theme. The German cutting center of Idar-Oberstein became the principal place for cutting tigereye soon after its discovery in the late 19th century. The Idar lapidaries, long adept at altering the color of agates, quickly discovered that they could bleach tigereye to an evenly colored light yellow using either hydrochloric or oxalic acid, although accounts differ as to which was best. (Note: These are both dangerous acids, and no one should attempt to use them without the proper experience, equipment, and facilities for handling dangerous chemicals.) When properly oriented and cut, this material could yield a sharp cat's-eye stone that was more than just a little reminiscent of real cat's-eye (an expensive variety of chrysoberyl; see Question 12). In the 1960s, we sold hundreds of these cat's-eye tigereyes, many to Bay Area street artists who did well selling them in inexpensive rings. As demand increased, we began to have trouble meeting it. Then suddenly we could get all the stones we wanted again, and they were even better than before: now the line of light down the middle of the cab was absolutely straight. Everybody was ecstatic until, on a whim, we sawed one cat's-eye in half - and it turned out to be a composite. Two pieces of straight-fiber, bleached tigereye had been glued together so that the chatoyance of each side met in a perfect line. So ended the cat's-eye tigereye boomlet in Berkeley. Tigereye can also be bleached beyond yellow to a gray stone that aniline dyes can turn any conceivable garish color. Though popular in the 1950s and '60s, these tigereyes have now, fortunately, faded from sight almost as completely as their colors faded after a week or two of exposure to sunlight. |
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No one seems to know for sure, and we suspect that the haziness of its history is related to some shady dealings when the material was first introduced to Europe and the U.S., especially since it may have been confused (deliberately or otherwise) with cat's-eye (see Questions 11 and 12). It seems pretty certain, though, that tigereye made its gemstone debut sometime in the late 19th century. Interestingly, it fetched several dollars per carat back then. A 1932 German book on gemstones (Bauer and Schlossmacher, 1932, 674) states that in about 1880, tigereye sold for 25 to 39 Marks per carat, or between $6.25 and $7.50 per carat - at a time when western American miners usually fought in vain to earn $3.50 for a hard, dangerous, 10-hour day underground in rich gold and silver mines. |
Tigereye is currently preferred by Americans and tiger's-eye by people who speak English (as Anglophiles might say). In the past, American publications have used a hyphen in both tiger-eye and tiger's-eye; the South African government refers to it as the latter. An earlier but now discarded term, griqualandite, has also been used as a synonym for tigereye. |
As described in Question 10, this is a silicifed crocidolite in which the original blue color of the crocidolite and sometimes some of the original crocidolite has been maintained. It is also called falcon's-eye (or Falkan-Auge in German, worth noting because before World War II, and even after, most tigereye was cut in Idar-Oberstein, Germany, and German scientists wrote most of the gemology textbooks). Fine pieces that have been polished well can be quite attractive, particularly set in silver, and are generally more expensive than tigereye. However, in our experience selling hawk's-eye rough, we found that some pieces weren't totally silicified, making them difficult to sand and polish properly; even when well polished, it's simply not as bright and colorful as tigereye. The presumably smaller supply of hawk's-eye presumably accounts for its higher asking price. Some material on the market is part hawk's-eye and part tigereye, cases of mineralogical arrested development, you might say. These partly blue, partly golden chatoyant stones are particularly interesting, and we found that they were much better sellers than either solid tigereye or solid hawk's-eye, although the fact that the University of California at Berkeley's colors are blue and gold might have helped skew our sales in Berkeley. Many reference books state that such mixed color material is called zebra or zebra tigereye, but we've never heard anybody in the trade actually use this term out loud. Ditto for the term bullseye to refer to the red, treated tigereye (see Question 11). We would have to regard both of these terms as belonging to some other aspect of the bull! And while we're on peculiar names, sometimes tigereye is found where all of the color-producing minerals have been removed, leaving only white polycrystalline quartz. With a surprising lack of commercial imagination, such material is called quartz by South African dealers. |
Some tiger-iron is all golden just like the South African tigereye, and this kind is also called Australian tigereye 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 tigereye jasper. Some also incorporates silvery gray hematite; this is called tiger-iron by pretty much everybody. Western Australia is home to some of the world's greatest iron ore (hematite) deposits. Associated with these are huge areas of jaspillites (a jaspillite is a hematite-rich jasper) and banded siliceous ironstone. Sometimes fibrous crocidolite is also associated, and sometimes it has been replaced by silica, just as in the tigereye deposits in South Africa. Australian tiger-iron or tigereye was reported by R. O. Chalmers as early as 1968 (312). According to B. Myatt (1974, 182), deposits of varying grades of tiger-eye have been found in the Brockman and Marra Mamba Formations of the Hammersley Ranges, in the north-west province of Western Australia. He also reported another occurrence as near Cowell in South Australia. |
In the late 1990s, pietersite that was supposedly from Henan Province, China, appeared on the market, which we described in our report of the 1997 Tucson shows (We Predict, May 1997). We also described our surprise verging on doubt about the locality (the material looked just like the Namibian pietersite), and in response received an informative letter from geologist Wenlong Liu of the Great Wall Consulting and Trading Company in Northglenn, Colorado, reproduced in part here. The pietersite was originally found (as a part of asbestos mines) at Neixiang-Zhechuan counties, Henan Province, in the '50s. No one considered using it to make jewelry or carvings for decades. People in China have no idea pietersite has been popular outside of China. Starting in 1993, people in Henan province dug out some (called eagle-eye in China) and made bracelets and other handicrafts. At the beginning, the stuff was very cheap. People loved the new material. In the first couple of years, it was sold mainly to Hong Kong and Taiwan. In 1996 and 1997, two companies, including the Chinese firm Treasure Kingdom Museum, brought it to Tucson. The Chinese pietersite is very similar to the Namibian one, but it has more red color and distinctive golden-red combinations. As the material gets popular, the source is being exhausted. It contains both chrysotile and crocidolite fibers. Petrification is very strong, so that no harmful free fibers exist in most samples. At some spots miners dig so deep that they have encountered groundwater problems. |
In their Standard Catalog of Gem Values (1994, 86), Anna Miller and John Sinkankas state under tigereye that binghamite is a tigereye from the iron deposits of Minnesota; its fibers are much finer and chatoyancy sometimes greater. Silkstone is analogous to binghamite but the fibers are randomly oriented and it is not so highly prized. |
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In what distant deeps or skies As much as we would like to believe that William Blake was inspired by a tigereye gem, we know it isn't too likely, especially since tigereye was unknown in his time. Poor fellow, he never knew what he missed. |
Acknowledgments
Bibliography
Bauer, Max revised by Prof. Dr. Karl Schlossmacher (1932) Edelsteinkunde. Leipzig: Bernhard Tauchnitz. Si & Ann Frazier have been in the gem, mineral, and jewelry supply business since 1965 and are currently working on The Encyclopedia of Quartz. Si has also taught gemology, mineralogy, and related courses at the university level, and is a lifelong rockhound.
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