All these stones are silicates of aluminum and one other metal, either potassium, sodium, or calcium, but these may be intermixed so that chemically there is no strict dividing line between the varieties. The potassium feldspars are grouped under the name of orthoclase, the others as plagioclase. These two groups crystallize in two different systems, the former monoclinic and the latter triclinic, although the two kinds of crystals resemble one another. For jewel stones, all the feldspars are rather soft, being only 6 on Mohs' scale; specific gravity is about 2.57, and refractive indices are 1.51-1.52. Moonstone and amazonite are both feldspars, yet they differ vastly in appearance. Moonstone is almost transparent and colorless, although the clear and pale, milky white stones are not very attractive and have little value. Those showing a decided bluish opalescence, which varies as the stone is moved, are considered the better qualities. The whitish stones showing no opalescence are valueless. The material is never faceted, cabochon forms, often with a high table, being used. The best effect is thus obtained, since the sheen appears in certain directions only. The opalescence is caused by the reflection of light from the very thin layers of material which make up the stone. The luster is vitreous and pearly, and the cleavage is good. Cutting and polishing is therefore not difficult, and most of the stones we see have been fashioned in Ceylon by the local lapidaries, for Ceylon is the chief source of supply of this stone. Good crystals are seldom found; these have cleavage planes which run mainly at right angles to each other, but many stones are found as pebbles. The gem gravels and quarries of Ceylon yield a number of moonstones, and these are cut and exported in considerable quantities, shapes and quality being no bar to the industry of the local cutters. Notwithstanding this, large fine stones are comparatively rare and are sold by the carat. Other stones occur as rough fragments in a clay which may have resulted from the decomposition of an igneous rock. Clear, colorless crystals are known as adularia, and some such material has also been found in Madagascar as well as in the St. Gothard district of Switzerland. Although moonstones are often sold for a few pence, they are sometimes imitated by a clouded glass. The imitations generally show internal bubbles of air and, if tested, do not show double refraction like the true stone. There is a milky type of quartz found in Madagascar, pieces of which have been cut, and these resemble moonstone also. But this material lacks opalescence and it is also slightly harder than moonstone. As the general color is neutral, moonstone tones well with many other colored stones and, if mixed with discretion, makes up into very attractive jewelry where a number of stones is required. Drilled beads of good quality and size, when in necklace form, realize higher prices than those of the quartz group. Moonstone is regarded by many as being a lucky stone, and this belief persists on account of the efforts of the vendors in the East. Amazonite, or amazon stone, is opaque and green to bluish-green in color. White streaks and cracks, the latter often brown, spoil many otherwise good pieces. The color somewhat resembles certain jadeites and chrysoprases, but the green has a moving sheen caused by the thin plates of which it is composed, or possibly it may be due to small included cracks. It also does not take a high polish and is decidedly softer than jade, chrysoprase, and aventurine, with which it is sometimes confused. The incorrect use of the term "amazonite jade" also helps to give the general public a wrong impression of the nature of this stone. Pale specimens are seldom cut since they have no attraction as ornamental stones. The material is seldom faceted; cabochon forms are most common; but since it is very easily cracked or broken along its cleavage planes, it is not very suitable for many purposes. The green color may be the result of a small amount of included copper and organic matter in its composition. Many specimens certainly fade in course of time, but this may be due to their being artificially treated to improve their color before being marketed. This is done in Idar-Oberstein, where most of these stones are cut and fashioned. As with fluorspar, a grease is often used in the working of amazonite to prevent its splitting or cracking, and this foreign matter sometimes becomes evident in brownish marks, particularly where cracks and faults occur in the material. Well formed crystals, belonging to the triclinic system, are found in cavities at Pike's Peak, Colorado, U.S.A., as well as in Canada (Ontario), Brazil, and the U.S.S.R. (near Lake Baikal and Chkalov). The origin of the term "amazon" in connection with this stone seems to be the result of a misconception since the rough material is not found in the neighborhood of the Amazon River. Although not widely used, ancient ornaments of this material have been found from time to time, which shows that it has been recognized as being of some value for many centuries. Its use in jewelry for many years has been very restricted, and we have seen it mainly in the form of drilled beads in necklaces. Labradorite, or Labrador stone, belongs to the plagioclase group, masses or blocks of the material being found, associated with hypersthene, along the coasts of Labrador. An essential constituent of certain basic igneous rocks, the material is remarkable for its play of colors. Although normally a grayish-blue opaque stone, green, red, and yellow broad flashes may be seen on the stone being moved. This play of color determines its quality since dull, black, or gray material is of no use in jewelry. The play of color, which resembles poor black opal to some degree, is probably due to interference of light from the twin lamellar inclusions which are generally present, but it may be due to polarization caused by the lamellar structure of the material. The various broad flashes of color, particularly the peacock blues, make this a very attractive yet cheap ornamental stone. Its softness, however, is a great disadvantage. Si ones are cut in cabochon forms, with large flat faces. The luster is rather greasy, and a high polish cannot be obtained. Softness and easy cleavage detract from its use and, unfortunately, the discovery of huge quantities of rough material in North Russia as well as in Labrador has lessened its value considerably. Nevertheless, its beauty remains, and it could be used more extensively in cheap jewelry where larger pieces of stone are required. When first discovered in Labrador, the stone was considered of some value. Missionaries who had visited the Eskimos introduced it into Europe, and for some time there was no other source of supply. Since the discovery of large deposits in North Russia and elsewhere, there has been a great decline in the demand for this stone and it is now seldom seen in jewelry. Pendants, cameos, and beads, all cut in flat cabochon forms, are sometimes marketed. Although the best material still comes from Labrador (in the vicinity of Nairn) and the nearby island of Nopoktulagatsuk, stones suitable for ornamental purposes are also found in northeast Quebec, Canada. Labradorite may be sometimes confused with Blue John, a fluorspar. The colors and appearance are similar, but Blue John does not show a play of color and it generally possesses an amethyst tinge. It is softer than labradorite, and also shows single refraction. Some labradorites also resemble bluish opals, but here again, color is the only similarity. Sunstone, or aventurine feldspar, is very seldom seen in jew elry. It is generally a reddish stone, but colors vary, being at times brownish, whitish, or even greenish. The spangled effect it shows, which is its chief characteristic, is due to naturally included flakes of hematite or a similar material. This quality sometimes causes confusion between sunstone and aventurine quartz. The latter is a harder stone and is generally of a dark green color with black spots. Sunstone is a sodium calcium plagioclase; crystals belong to the triclinic system, but the material is generally found massive. Twinned crystals are not uncommon, however, and cleavage is perfect. The chief sources of supply are India, Norway, Finland, and Czechoslovakia (Bohemia). The artificial "goldstone" should not be confused with sun-stone. This is a hard glass, generally brownish in color, with a mass of closely set golden specks caused by copper crystals or copper filings, which have been introduced evenly over the whole material before cutting. The artificial product is in more general use than the natural stone. Non-gem feldspar, which is a combination of orthoclase, micro-line, and albite, is used in the manufacture of glass, more than half of the total world production of this mineral being consumed for that purpose. Large quantities are also used in the ceramic industry, and it is an important constituent of both the body and the gloss of porcelain and china. Such material comes from the U.S.A. (North Carolina, South Dakota, and New England), Canada (the provinces of Ontario, Quebec, and Manitoba), Sweden, Norway, Scotland, and Cornwall.
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The feldspar group of minerals are important inasmuch as they are widely distributed and enter into the composition of many rocks. Only a few of them are suitable to be cut and used as jewel stones and, owing to their abundance, none is of great value. Perhaps the best known to us is the moonstone; others are labrador-ite, amazonite, and sunstone. Each has its particular attraction, but the moonstone is the most popular, being used extensively in the cheaper and medium priced jewelry. Large stones of this species are not common, especially those showing a fine, bluish opalescence, and these command a fair price.