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#1 ha_huong


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    Posted 04 December 2010 - 07:59 PM

    anh Cunin ơi, anh có biết hiệu ứng sao  và hiệu ứng mắt mèo trong peridote không ạ?
    ở Việt Nam mình thì Peridot đặc biệt đc thành tạo trong các hốc Bazan phải không ạ?
    Anh có biết nhiều về nguồn gốc Peridot của Việt Nam ko ạ?
    Công ty TNHH MTV Vàng bạc đá quý Ngân hàng Sài Gòn Thương tín
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    #2 cuniu



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    Posted 06 December 2010 - 09:05 PM

    Những viên peridot có hiệu ứng sao hoặc mắt mèo thì rất hiếm. Muốn tìm hiểu về nó thì đầu tiên bạn phải hiểu được cơ chế tạo sao trong những loại đá có hiệu ứng sao phổ biến như ruby, sapphire, từ đó bạn tìm hiểu về cơ chế tạo sao trong peridot và viên đá phải được cắt mài như thế nào mới tạo ra sao. Mình gởi bạn một số bài viết về hiệu ứng sao và mắt mèo của peridot bạn cố gắng dịch còn hình thì mình gởi sau. Về 2 câu hỏi còn lại thì mình nghĩ bạn nên cố gắng tìm hiểu qua sách vở hoặc thầy cô vì bản thân peridot cũng là olivine màPosted Image.
    PERIDOT, Cat's-Eye Manyyears ago, Webster stated that 'A cat's-eye peridot is known," butprovided no additional information. In his Color Encyclopedia of Gemstones, Aremreported that "cat's-eye and star peridots are known, but are very rare."Perhaps one reason that we encounter phenomenal peridot only very rarely isthat the preferred cutting style for this gemstone is faceting rather thancabochon.

    A long-time friend of CIA recently gavethe Los Angeles laboratory the opportunity to examine the very interesting15.73-ct cat's-eye peridot illustrated in figure 9. Even to the unaided eye, itwas readily evident that this oval cabochon is filled with small dark browninclusions, so much so that the overall body color of the gem is affected. Examinationwith the nlicroscope revealed that these inclusions are ultra-thin and varygreatly in size and shape; some are rectangular, while others exhibit dendriticformations. John Koivula suggested that they are most probably an iron compoundclosely related to ilmenite (Fe +2TiOi). The dendritic appearance of many ofthese inclusions is characteristic of both rapid crystallization and anexsolution product. Their orientation in definite crystallographic directionsgives rise to the chatoyancy. At certain viewing positions an additional,weaker ray is seen, creating a star with one prominent ray and one vague ray.Oriented exsolution ilmenite together with hematite is also believed to be the causeof asterism in some star beryls.

    Interplanetary cat’s-eye peridot. Pallasite,a type of stonyiron meteorite first described in the 18th century, is known forthe yellowish green olivine that can be extracted from it. Yet pallasiticperidot, the gem variety of olivine, is extremely rare. (For historicalbackground and a gemological examination of nine faceted samples, see J. Sinkankaset al., “Peridot as an interplanetary gemstone,” Spring 1992 Gems &Gemology, pp. 43–51.) At the 2008 Tucson gem shows, meteorite hunter Steve

    Arnold of Kingston, Arkansas, showed the G&G editors fivepallasitic specimens: one faceted peridot, one oval peridot cabochon showingchatoyancy, a rough piece of peridot, an irregularly shaped cabochon, and aslab of pallasite containing gem-quality peridot (figure 31). Using a metaldetector, Mr. Arnold discovered several kilograms of the material in 2006 neara known meteorite location in the United States. He took the rough to RickStinson (Stinson Gemcutting Inc., Wichita, Kansas), who observed that some ofthe peridot was chatoyant (figure 31, inset), a phenomenon that is very rare interrestrial peridot. According to Mr. Arnold, the American Museum of NaturalHistory in New York later identified the cause of chatoyancy as parallel,tube-like hollow inclusions. Peridot is a relatively soft gem material (6.5 onthe Mohs scale), and the pallasitic material seems more fragile than peridotmined on Earth, perhaps due to the stress of its passage through the atmosphereand subsequent impact. In fact, a small piece of the peridot cabochon chippedoff as the stone was being prepared for photography in Tucson.

    Because extracting thegem-quality peridot is so difficult and destructive, Mr. Arnold estimates thatless than 1% of the total weight of the recovered meteorite material will beconverted into finished gemstones. So far 40 stones have been faceted, rangingfrom 0.20 to 1.04 ct, and only a few cabochons showing chatoyancy have beencut.

    Rare Star PERIDOT

    Cat’s-eye peridot israre, and star peridot is rarer still. Only two examples of the latter havebeen reported by the GIA Laboratory (Spring 1960 Highlights at the Gem TradeLab in Los Angeles, p. 3; Summer 1987 Lab Notes, p. 106). The first wasdescribed as having a “well-defined four-rayed star reflected from tinyneedlelike oriented inclusions,” the second as having a star with two strongarms and two weaker arms. Recently, we were loaned for examination a 22.21 ctoval peridot with a uniformly distinct fourrayed star (figure 10). Standardgemological testing confirmed the stated identity and gave the followingproperties: diaphaneity/ color—transparent to semitransparent brownish green;spot RI—1.65; hydrostatic SG—3.32; and absorption bands observed at 453, 477,and 497 nm in the desk-model spectroscope. Despite the stone’s relatively hightransparency, examination with a gemological microscope revealed that it wasfilled with inclusions. “Fingerprints” and brown platelets were the mostobvious internal features (figure 11, left). With fiber-optic illumination, orientedfine iridescent thin films, tiny needles, and short strings of reflectiveparticles were observedthroughout (figure 11, right); these abundant inclusionswere the cause of the asterism. We performed Raman spectroscopy in an attemptto identify the platelets, but the spectra showed too much interference fromthe host peridot. As many of the platelets appeared brown in transmitted light,the possibilities include phlogopite, biotite, or even the ilmenite that was suggestedfor the peridot in the Summer 1987 Lab Note. Accurately photographing the asterismproved quite challenging. As a result, the image in figure 10 is a composite oftwo digital photos—one focusing on the star, the other on the outline of thecabochon.

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