Chalcedony
Posted on June 17, 2008
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When properly used, this term includes a group of quartz minerals, all of, which are distinguished by having no visible crystalline structure. The composition is of a fibrous nature, and under the microscope, a mass of very small individual crystals can be determined. These are doubly refracting, and other physical con-slants are about the same as the crystalline varieties of quartz, that is, hardness is about 7, and specific gravity 2.62 to 2.64.
Most of the stones are either translucent or transparent, the Colors varying considerably. But since the material is porous, it lends itself admirably to staining and alteration by various chemical processes. Many of the familiar colors are not natural, some having been accentuated, others altogether introduced by various means. Agates and jasper are frequently stained, while carnelie ban and sard are also generally altered before being marketed, the resulting colors being usually much stronger and deeper than that of the natural material.
This artificial alteration leads to some confusion in names, the general public knowing certain colored chalcedony by terms which are incorrect. For instance, a green stained chalcedony is called chrysoprase, whereas there is a natural chrysoprase, and the pale, bluish stained material is known simply as chalcedony, which is also incorrect, since this is the name for the whole group. The term itself is derived from the ancient city of Chalcedon in Asia Minor, which is not far from the present Scutari, and where large deposits of the rough stone may still be found.
We will now detail some of the more important varieties of chalcedony, commencing with agate, which is both widely worked and used.
Aventurine
Posted on June 17, 2008
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Also found massive, this is an opaque, dark green stone, impregnated with blackish spots, these being naturally included mica and hematite. Inspection with a microscope would reveal the metallic luster of these inclusions, and a strong light would produce a spangled effect, although normally these effects are not seen.
The cut stones, which are very seldom faceted, are usually of a uniform dark green color, opaque, and spotted by inclusions, but seldom flawed. Faulty material is not worth cutting, and there is little demand for this stone in jewelry. Yellow and reddish-brown aventurine are also found, but none of the shades is very attractive. As with rose quartz, large pieces of good quality are not abundant, yet commercial values are low. Sunstone, a variety of feldspar, is very similar to aventurine in appearance, but it is a softer stone. An imitation of the reddish-brown aventurine is often seen, this being made of molten glass into which flakes of copper have been introduced. These flakes give the required spangled appearance, the body color usually being of a coffee brown. It is said that this imitation, called Gold-stone, was first made by accident, some copper filings being upset into some fused glass. Although fairly hard, it is softer than natural aventurine. Blocks of the material are made up in Italy, where some is cut and mounted into cheap jewelry, but more is sent to Germany, where it is fashioned into ornaments for tourist trade. This imitation has a finer effect than the natural stone, the included metallic flakes being more numerous and brighter. The process of manufacture is said to be a secret, some experience being necessary to obtain the required conditions which produce this glass. But it is generally composed of eight parts of glass, one part protoxide of copper, and two parts of oxide of iron heated together for some time and then allowed to cool very slowly.
India, Brazil, and Siberia (Altai Mountains) produce the rough stones, and much is carved in India. It is sometimes combined with diamond engagement rings. Hence the name “Indian Jade” for the green aventurine, and visitors to India and China might be persuaded to buy this cheaper stone for jade. Carved ornaments and beads are made, and the Chinese hold this stone in high esteem. In Western countries, it is regarded as being of little value.
Rose Quartz
Posted on June 17, 2008
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As its name implies, this form of quartz is rose pink in color. Very often flawed by internal fissures, poor qualities are almost white with only a suspicion of pink. Large pieces of good quality, with a definite deep pink shade, are rare. Yet despite this, the value of the stone remains low on account of its very limited demand. Perhaps the color appeals to few.
The material is found in masses, and rarely as crystals. Generally opaque, there are some stones which cut into translucent and almost transparent pieces. Color is usually homogeneous, and if patches and cracks in certain stones are more deeply colored, it is certain that these have been stained artificially.
Unfortunately, rose quartz fades in color on exposure to strong sunlight, and also when heated. The cause of this change is not certain, but it is said that the original shade may be restored after some time if the specimen is kept in darkness and in a damp atmosphere.
Cut stones are rarely faceted, cabochons and drilled plain beads being mostly used. Larger pieces are sometimes carved by the Chinese into ornaments. Some of the material found in North America shows asterism and chatoyancy when suitably cut. In recent years, these so-called star stones, star chalcedony, or star quartz have been marketed in some quantities, and many have been mounted in jewelry. They are usually thick oval or round cabochons, and the six-rayed star effect is accentuated by a piece of colored glass or metal being fused to the base of the stone. As they are made to simulate good quality star sapphires, the artificial base is colored blue, but there is no reason why any color should not be produced.
Formerly, a triplet was made, consisting of the rose quartz with a blue chalcedony base, and a thin blue glass, piece of metal, or mother-of-pearl between, all the edges being ground flush. The star effect is best seen under a strong direct (not diffused) light, and the star seems to move as the stone is moved, much the same as in star sapphire. There are, however, several obvious differences between these two stones. The latter is harder, with a higher specific gravity, and the star effect is seldom so perfect or strong as in the star quartz. The blue backing gives the latter stone a glassy or artificial appearance.
The chief sources of supply of the rough are Brazil, Madagascar, U.S.A. (Maine), Bavaria and the Ural Mountains.
Cat’s eye
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Cat’s eye has a peculiar internal structure which gives a chatoyant effect if the stone is cut in cabochon form. A moving play of light in one direction is seen, like a luminous line or band. This is due to the parallel, minute fibres of asbestos within the stone, and in this way, it resembles chrysoberyl cat’s-eye. The latter stone is, of course, much harder and has a higher specific gravity than quartz. But great care should be taken to distinguish between these two varieties since the quartz cat’s-eye is of much less value than the other stone. Also, it should be noted that the term “cat’s-eye” is used loosely in the gem trade, and mistakes are quite commonly made.
The natural color of quartz cat’s-eye is an opaque greenish or greenishbrown, but quite often the material is artificially stained, dark blues and pink being common. These colors are not permanent. Seals, the handles of sticks and umbrellas, boxes, as well as the popular forms of diamond jewelry often contain cat’s-eyes, the rough being found in Ceylon and India, where the stones are regarded as having some value. Some inferior material is found in Bavaria (Germany), and these are the socalled Hungarian cat’s-eyes, none being found in Hungary. The stone is often confused with tigereye, another variety of quartz, which is much more abundant. Read more
Amethyst Origin
Posted on June 17, 2008
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Great quantities of amethysts have been mined in Brazil, although in recent years supplies show signs of exhaustion. In the southern state of Rio Grande do Sul, there are many worked deposits, which also yield agates. In Minas Geraes and Espirito Santo, stones are found in veins in pegmatite dykes, while the best qualities come, at present, from the Brejinha Mine in Bahia. These are uniformly colored and of a deep purple shade. Light colored stones, found as waterworn pebbles, also come from Bahia (Xique Xique), and this state has produced some remarkable specimens in the past, both in size and perfection. One crystal, found in 1928, weighed 206 lbs and was 25 inches high. Several stones of good quality were cut from this specimen in Idar-Oberstein during the same year.
The once fashionable practice of engraving amethysts for seals and jewelry pendants is now almost obsolete. There is little demand for such work, and the skilled craftsmen who specialized in this do not now exist. But stones for rings, brooches, and pendants are CUt in good quantities, and they are still popular in jewelry. Their use in necklaces varies with the trend of fashion.
As we have noted, the discoveries of large quantities of good quality rough material in South America during the last century resulted in the fall in the value of the stone, and now ordinary stones are valued at only a dollar or so. In the year 1625, it was recorded that an amethyst was worth as much as a diamond of equal weight. At the beginning of the Nineteenth Century, $5600 (£2000) was paid for an amethyst necklace belonging to Queen Charlotte of England, but now this would scarcely realize $560 (£200). Yet amethyst was always regarded as a precious stone, and in Biblical days, it seems that it was highly prized. It is frequently mentioned in the Bible, and there are also various old superstitions connected with its supposed properties, the chief being that it serves as a guard against drunkenness. Hence its name, from the Greek a (not), and metho (to be drunk).
There is a good demand for older types of mounted jewelry which contains fine quality amethysts, and prices for these have advanced in recent years. Good quality stones of some size are not common, but small stones may be had in abundance. Amethyst is the only purple colored gem stone which is found in quantity, others, such as purple sapphire and purple spinel, being comparatively rare.
Amethist Abundance
Posted on June 17, 2008
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Madagascar stones, which are of a violet red tint, often deeply colored, are distinctly dichroic. When heated, these stones do not change to a brownish-red color as do the South American stones; in fact, they change into a white quartz of different crystal symmetry at 5750 C, finally melting at 17100 C. Deeply colored stones, which are the most sought after, are generally particolored, flawed, or feathered, and a bright, transparent, deep purple red stone without flaws and of homogeneous shade is not at all common, despite the abundance of amethysts seen. In fact, fine specimens are scarce and they are getting scarcer, yet $2.80 (£1) per carat would be a high price for an amethyst.
The deep purple color is, in many instances, probably due to included manganese oxide, but this may not be the only tinctorial agent. It is, in fact, now attributed to radioactive radiations since the purple color may be restored by beta and gamma rays. Others think that a combination of ferrous and ferric oxides may be the cause. Read more
Amethyst
Posted on June 17, 2008
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Of all the mauve stones found in nature, this is the most abundant, yet a fine quality amethyst is comparatively scarce. The deeply colored purple specimens are very beautiful, and they blend well with many other colored gems. Amethyst may therefore be used with advantage in combination with diamond, emerald, peridot, moonstone, and other gem stones, and its popularity is proved by the many pieces of diamond jewelry of all ages which exist and which contain specimens of this stone.
In color, amethyst varies from a faint violet to a deep purple, different mines often producing distinct shades. Pale yet bright shades are common in the Brazilian stones, and Uruguay produces the deeply colored, but often faulty, material. In the Urals are found the darker colored specimens, very brilliant, but generally parti-colored. It is often possible to name the origin of an amethyst by its shade of color, although this is of no particular advantage. Read more
Citrine
Posted on June 17, 2008
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This name, which is not in general use among gem dealers, is really reserved for the pale yellow, transparent quartz, which is often of a fine, limpid color. Some pieces show zones of deeper shades, the variations probably being due to included traces of ferric oxide. Natural stones are fairly scarce, and the majority of those on the market have been produced by suitably heating smoky quartz. Color alone decides the name, and there is no strict line of demarcation between citrine and smoky quartz. Citrine, however, has a peculiar internal structure (lamellae in twin positions, but less intricately twinned than amethyst), and it is also distinctly dichroic.
Brazilian topaz, “topaz,” and very occasionally “occidental topaz” are terms which are loosely used with citrine, and it should be emphasized that the real topaz should be carefully distinguished from the quartz variety. The latter has various different physical properties from the true topaz, such as lower specific gravity, a lower grade of hardness, and lower refractive indices. It does not show the brilliance or take the very high polish of the more expensive gem. Read more
Smoky Effect Of Stone
Posted on June 17, 2008
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The smoky effect which is characteristic of this stone is due to the presence of sodium. Deeply colored specimens show distinct dichroism. The rough is found in the Cairngorm Mountains (Scotland), but in much larger quantities in Brazil, Uruguay, Spain (Cordova), and Switzerland. Some exceptionally large crystals were found in 1868 in Switzerland, and these may be seen in the museum at Berne.
Among gem dealers, topaz quartz is the name applied to worked material which has not been faceted, such as beads, cabo-chons, etc. It is generally flawed and of the cheapest quality, sometimes being cut to include pieces of parent rock.
Other faceted stones, often of a dark brown color and very much like certain garnets, are the result of heating selected amethysts of a poor quality. Some Uruguayan and Brazilian stones are “burnt” to produce these so called dark, reddish brown “topaz.” Such treated stones usually show no dichroism. Much of the smoky quartz from Minas Geraes (Brazil) is heated to produce citrine, a light yellow variety of quartz. Amethysts from a new mine have been treated Fairly recently to produce stones of a greenish color, which are cut locally and sold as “prasiolita.” Look at topaz diamond jewelry.
Smoky quartz may be seen mounted in cheaper forms of jewelry, but its sale is limited, except in a few countries. The lighter yellow varieties are very common, and these pass into the class of quartz known as citrine
Smoky Quartz
Posted on June 17, 2008
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<p>Smoky quartz is aptly named, for although the degree of color varies from the unusable black to a yellowish brown, all varieties of this stone show a darkish smoky effect. The smoky blackish stones are known as morion, the reddish brown and brownish yellow as cairngorm, or Scotch topaz (after the locality where some such material is found naturally), and the lighter yellow as quartz topaz, Spanish topaz, or just “topaz.” The use of the term “topaz” in connection with this stone is entirely incorrect and misleading, but it is freely used both within the gem stone trade and by the general public. Check also various gemstone rings. It should be applied solely to the precious topaz, a stone which the lighter yellow varieties somewhat resemble. </p>
<p>When a stone is offered for sale as a topaz, one should make sure what it really is, since the true topaz is more valuable than the quartz variety. In order to avoid the confusion which results from using so many different names for this variety of quartz, various suggestions have been made. These Suggestions have had little success, although it must be admitted that there is now a wider appreciation that the term “topaz” in this connection should be avoided.
Morion becomes colorless if strongly heated, but at a moderate temperature, it changes its color to a rich brown. Such effects are permanent, and the altered stones are difficult to distinguish from natural material. But commercial values are so low that this is of no great consequence. </p>