Chrysoprase
Posted on June 17, 2008
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Natural chrysoprase is of a fine apple-green color, somewhat resembling certain jades and amazonites. Duller, and of a different texture from jade, and without the tinge of blue and the pearly sheen which distinguishes amazonite, chrysoprase is seldom a deep green, imperfections being very common. It is often streaked with white, and cracks are quite usual. Finely colored pieces of size and without flaws are scarce, and these command a price above all other types of quartz, with the exception of fine amethyst. The green color is due to a small amount of included nickel oxide.
The color, however, is not stable. Exposure seems to have a tendency to bleach the stone, and heat will affect it to such an extent that it will change to a chalky white. This loss of color is due to loss of water, but the original shade may be sometimes restored if the stone is buried in moist earth, or in cotton wool. Some pale stones may be improved by soaking in a solution of nickel sulphate.
In the process of polishing, heat is developed, and unless great care is taken, cracks will develop and spoil the material. These properties render chrysoprase somewhat unpopular both to the lapidary and to the dealer, and as there are so many other green stones to be had, not much is seen in mounted jewelry.
Yet, as a gem stone, chrysoprase has been worn throughout many centuries, and it forms a part of many old works of art, mosaics, and inlaid work. Good stones are still valuable, and large, fine specimens have been known to sell at $2,800 (£1,000). But present demand is very little and the stone obtains a low price.
The Silesian mines date back to the 14th century, although they have not been worked consistently. For many years they were neglected, and in 1740, they were apparently re-discovered by a Prussian officer. Frederick the Great interested himself in the production of stones from these mines, and there are some-fine specimens in many German museums, the stone being more popular in Germany than elsewhere. The mines are in the Frank enstein, Glasendorf, Kosemiitz, and Baumgarten districts, all south of Breslau. Other localities where chrysoprase may be found are in the Ural Mountains, Brazil (Goiaz), and in some-states of North America, particularly in California, Arizona, and Oregon.
In California, chrysoprase occurs in seams and veins in the jasper rock and serpentine. It is often associated with common opal and an almost colorless chalcedony. Mining is generally confined to open work, and where shafts and tunnels have been made, they are not deep. Qualities range from the poorest to the-finest, the best specimens being highly translucent and of a fine-, emerald grass-green color. The material found in Arizona is nearly all of a poor quality.
Among stone dealers, it is quite common to- use the term chrysoprase in connection with the stained green chalcedony. This is quite incorrect and, unfortunately, it has been passed on to the general public. It will be remembered that chalcedony is porous, and that the green may be obtained by various methods, among which are by soaking in a nickel salt solution, or in a chromic acid solution, subsequently heating in both instances. The green color thus obtained may fade in time; sometimes a brownish or brownish-yellow tinge will appear, while some stones will take on a thick, cloudy, heavy green shade. On the other hand, other specimens will appear a fine green with the same translucency as carnelian, and they will retain their acquired color.
This incorrect use of the term chrysoprase has resulted in calling the real chrysoprase “natural chrysoprase” to distinguish be¬tween the two. The stone might not be bought so readily by the public if it were referred to as stained chalcedony. It has been marketed by some as green carnelian. Various shades of green are obtained, but they are all brighter, more highly polished, and the material is more translucent, than the natural stone.
Plasma
Posted on June 17, 2008
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This is dull, leek-green stone, often speckled with white (but not red) spots. Yellow markings are also sometimes seen. It is opaque to sub-translucent, and except for its markings, it is the same as bloodstone. The dark green color is due to inclusions of green earth (or delessite).
Prase is much the same as plasma, except that it is more translucent, and it may occur either as crystals or in crypto-crystalline forms. The name is derived from the Greek prason, meaning “a leek.” These stones are too uninteresting to be cut and used in jewelry, although they are occasionally seen.
The rough material is found in the eastern states of India, and in Germany.
Jasper
Posted on June 17, 2008
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Jasper is a massive, compact quartz, usually containing many impurities. Opaque, dark red, brown, yellow, green, or gray, the included ferric oxides, iron silicates, and clay give rise to the different colors, which are all rather dull. The internal structure is so fine that the constituents cannot be recognized except by analysis. Read more
Bloodstone
Posted on June 17, 2008
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Slightly more interesting than sard, bloodstone is also opaque, although its dark green color is relieved by small red spots. The better material is evenly distributed with these spots; that containing larger marks or stripes is considered to be inferior. Occasionally mounted in rings, sometimes carved, it is worn by some who consider it their “birthstone.” Sometimes it is referred to as heliotrope, but this name should be avoided to prevent unnecessary confusion. The rough material is found in some eastern parts of India, as well as in Australia, Brazil, and the U.S.A. (Wyoming). Check Diamond Earrings also.
There is a bust of Christ in a Parisian collection which is cut from this stone, and it is fashioned in such a way that the natural red spots resemble drops of blood.
Sard
Posted on June 17, 2008
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This is identical with carnelian, except for color, which is a deep brownish-red. The brown is pronounced, and it sometimes approaches black, but it is never attractive and is therefore seldom used in jewelry. Sometimes internal black specks may be seen, and some specimens improve on heating.
It is found, often with carnelian, over a wide area, India being the chief source of supply. The Greeks and Romans used this stone for engraved intaglios and seals.
Carnelians Origins
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Stones found and cut in India and China are not often treated, and they are therefore generally paler in color than those which emanate from Germany. Indian cut stones are usually irregular in shape; beads are drilled from each end separately and the holes are seldom true. On the other hand, Chinese work is usually good, many of their carvings being extremely fine. They also show much skill in taking advantage of the different shades within the piece of material being worked.
Very fine carnelian in quantity is found in Brazil, as well as in Uruguay, and it is generally accompanied by agate and amethyst. India, Queensland, the U.S.A., and Japan are other areas where good material is found. Stones cut in Germany from South American material are predominant on the market.
Mention of agate deposits has already been made, and the same mines in India produce carnelian. The waterworn pebbles are found in the vicinity of Ratanpur, Damlai, and Dholikuva, in the Rajpipla State, where they have been worked for many centuries. The carnelian pebbles have been derived from the disintegration of former trap beds in which they formed geodes and veins. As with agate, most of the stones found are light in color, often with a milky tinge, and they are sometimes heated to accentuate the shade. This work, as well as the cutting into beads, cups, and vases, has been carried on in Limodra for more than 400 years. In the 17th century, some of the work was carried out at Cambay, where the art is still practised. The value of the stones exported from this area averages about f 20,000 (£7,000) yearly.
Although a very moderately priced stone, carnelian is very suitable for cheaper jewelry. It is pleasing in color, it tones well with many other shades, and it is sufficiently hard to withstand wear without showing much adverse effect. Its long use in jewelry has resulted in its being connected with many superstitions; for instance, the Arabs believe that it stops bleeding if applied to a wounded part.
Some fine carved carnelians exist. One, the seal of Michelangelo, is preserved in Paris. This is only about half an inch in length, but it is exquisitely engraved, supposedly by Pyrgoteles in the time of Alexander the Great
Carnelian
Posted on June 17, 2008
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This is the popular translucent brownish-red or flesh red colored chalcedony, often seen in bead necklaces as well as in cabo-chon forms, mounted in rings, brooches, pendants, and earrings. Many stones are pale in color, and in addition to the reddish shades, milky white and yellows are occasionally seen, although these strictly speaking should not be called carnelian. The term comes from the Latin caro or carnis, meaning “flesh,” or from corneolus and carnevolus, on account of its resemblance to the color of flesh or to the fruit of the cornil tree.
Faint stripes are often seen within the stone, but if the stripes are very marked, such specimens should be called striped agate, or red agate. The darker brownish-red stones are really sard, so it will be seen that the three varieties grade into one another. Occasionally, ruddy patches mar specimens that are otherwise uniform in color; deeper shades of red are uncommon in nature, most of such marketed stones being the result of artificial treatment.
The natural color may be improved by various means since the material is porous. The rough stone is treated before being cut; it is heated to a certain temperature, according to the nature of the stone, and then treated with ferrous sulphate. There are other methods, but this is the one usually adopted in Idar-Oberstein. Some stones split and are entirely spoiled if care is not exercised; if too much heat is applied, the material crumbles to a white powder. The alteration in color seems to depend upon the amount of iron oxide within the stone, and the process calls for skill and much experience before success may be obtained.
Agate Cutting
Posted on June 17, 2008
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Now, cutting is carried out by steel laps, the edge of which is coated with silicon carbide, and although electricity has taken the place of water power, a few agate mills still remain. In these, the large grinding wheels revolve partially submerged in the running water, the rate of revolution being about three times per second. The cutter lies in a horizontal position and presses the stone against the surface of the revolving wheel. Some stones are finished on electrically driven laps, but the stone revolving wheels are considered to be more suitable for accurate work. Polishing is effected by hard, wooden laps, to which various materials have been applied.
Besides being extensively used in cheaper forms of diamond jewelry rings, agate bearings are used in delicate balances for supporting the steel knife edges and the pan holders. The cutting of such bearings in agate for various instruments is of some importance, and this branch of the lapidary industry was developed in England during the war. Read more
Agate Origin
Posted on June 17, 2008
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Agate is found in cavities, often irregular in shape, and also in holes in old lavas. It has resulted from the deposits of silica being thrown out of solutions, the shape of the bands following the shape of the cavities. Often, the layers suggest the outline of an almond, many beautiful designs being seen.
Agates are found as rounded pebbles in some areas. In India, for instance, where they are locally called akik, the ground is worked by digging pits to a depth of 20 to 70 feet. These pits cave in during the wet weather, so that fresh excavations are made each season. This area, situated in the Rajpipla State, has been worked for a great many years, and Limodra and Cambay are the two centers in which cutting and treating the stones for color are concentrated.
Occasionally, agates are found which show a remarkable display of colors. These are called iris quartz, or rainbow quartz, and such material has been found in Oregon, U.S.A. The stone is made up of a great number of alternately placed bands or layers, and these act as a prism, thus producing the beautiful range Of spectrum colors. Such effects are quite natural, although they are not evident until thin slices are cut and polished. Considerable skill in cutting is required so that the brilliant effect is brought out to the best advantage.
Another peculiar type of agate is sometimes found in Burma, near Mount Popa, some 85 miles north-west of Malanda. These stones are a pure, opaque white, occasionally tinged with black and yellow, or with alternately shaded bands. They are found in the neighboring volcanic alluvium. When some of these stones are cut across the bands, they show a figure which resembles a Burmese pagoda, and good specimens of such a nature are sold at appreciable prices in the country of origin. The stones are worked at Kyaukpadaung and find their way to religious shrines, not being used as personal ornaments since, to the Burmese, the pagoda is an object of religious veneration. Fine specimens of these “Pagoda stones” are rare. Some are artificially colored, like the ordinary agates. They can be used for 1 ct engagement rings.
On account of the varying degrees of porosity of the different layers, artificial staining results in a deepening or total alteration of the natural pale shades. Deep blues and greens, as well as the black and white onyx, are obtained by chemical treatment, and the rough material which is the most suitable for staining obtains the highest price. The staining and treatment of agate is an important industry in Idar-Oberstein, to which German district it is almost solely confined. The originators of this process were Italians, but the secret of the treatment is said to have been conveyed to a German by an Italian while both were imprisoned in Paris. The Germans have since developed and extended the methods, and these call for much patience and skill.
The natural occurrence of the rough stone in the Nahe valley was of great assistance in the furthering of the growing industry, but the local material was rapidly exhausted. The discovery of new finds in Brazil by German emigrants came at an opportune moment. Huge quantities of splendid material were shipped to Germany, and these and other varieties of quartz have been sent at almost regular intervals, except during time of war. Remarkable deposits of magnificent sardonyx are found in Brazil, although the largest agate diggings at present are probably those in the Catalan district of Artigas, Uruguay.
Before the war, the rough stone was shipped from South America to Hamburg, and then sold by auction in Idar-Oberstein, stone merchants being the chief purchasers. After sorting according to market requirements, the stones were given out to workers, who shaped, polished, and drilled the material as directed. The merchants then had the finished stones mounted as jewelry, cabochons in rings, brooches, etc., but larger pieces and slabs were made into inkstands, blotters, clock faces, knife handles, calendars, and similar articles.
The actual lapidaries seldom marketed the finished stone direct, their work being regulated and paid for by the merchants. Their trade was world wide, and agate mounted goods were exported to every country. They found much prominence in coastal and spa resorts, where tourists and holiday-makers were often offered these articles as being made from local stones. Besides Brazil and Uruguay, many fine agates come from India (the Deccan region), Canada (Ontario), and the U.S.A. Fine moss agates also come from India.
Agate
Posted on June 17, 2008
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This variety is essentially striped, the stripes or bands varying in width and color, according to the conditions present during formation. The stripes may be distinct or faint, but they can generally be seen under a microscope. The differently colored bands may be wide or narrow: a stone examined by Sir David Brewster was found to have over 17,000 definitely marked bands in an area of one square inch. The bands run parallel to each other, straight, wavy, or zig-zag, but they are generally concentric.
Colors may vary within one stone, but they are usually pale, the commonest shades being milk-white, yellowish, reddish, and brownish. Bluish and greenish colors are unusual, and those usually seen in some form of jewelry are almost sure to have been artificially stained. The color may be irregular on account of included impurities. People who buy diamond engagement rings are often interested in agate jewelry also. Read more
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