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The three intersecting bands of light produce a six-rayed star. On rare occasions, asterism is seen in garnets, but here the star is four -rayed, garnet being of the cubic system. A cat’s-eye effect, known as chatoyancy, is seen in certain stones, particularly in the chrysoberyl and quartz cat’s-eyes. The chrysoberyl cat’s-eye, also known as the oriental cat’s-eye and cymophane,
Is a cloudy grayish-green stone which, when cut with a rounded surface, shows a pearly white streak. This is caused by internal minute parallel cavities sometimes filled with a liquid in the crystal, and reflected light produces the white sheen. The quartz, or Hungarian, cat’s-eye gives a similar effect, but here the cause is due to parallel fibers of asbestos, which are often greenish in color, being present, and reflection of light results in the band of light at right angles to the fibers. Tiger-eye, or crocidolite, as it is often called, shows a similar streak of color as the stone is moved. This stone, a type of amphibole found in Griqualand West, South Africa, is really a beautiful asbestos in which the iron has been gradually replaced by silica, a natural process known as pseudomorphic replacement. Crocidolite is properly the name of the blue asbestos. Tourmaline cat’s-eye is rare, but specimens have been found.

The “orient,” or sheen, seen in pearls is a play of color on a minute scale. Pearl is composed of thin, translucent layers of carbonate of lime and organic matter arranged concentrically. Light is thus diffracted and reflected, giving the attractive “orient” so esteemed in the best pearls. The color and the quality of the “orient” depend upon the composition, thickness, and arrangement of the layers.
Opalescence, strictly applied to the pearly or milky appearance seen in some stones, such as moonstone, is also caused by the material being made up of very minute thin plates, from which light is diverted and reflected. Moonstone, an almost clear stone with a milky sheen, shows this effect to advantage if it is cut in a curved form (en cabochon), with a high front. A bluish-white moving streak is seen in the best qualities, which is sometimes called adu-larescence.
Certain varieties of gem stones, but chiefly rubies and sapphires, may be found which show a moving star within the stones when they are suitably cut. This star effect, or asterism as it is called, is caused by a series of microscopic canals or hair-like mineral inclusions   (generally rutile) lying in the crystal and which follow the course of its axes. For instance, star rubies and sapphires,
when cut en cabochon with high curvature display a six-rayed Nlar of white light near the summit of the stone. The reason for
this is that the particular specimens contain three sets of parallel canals or inclusions, intersecting at an angle of 6o° and at right
angles to the vertical crystal axis, from which light is reflected.

The effect on many gem stones when X-rays are allowed to fall on them is interesting and in some instances could be used as a distinguishing test. The action of these rays on stones was investi gated by Dolter, and later by Bragg and others. They found thai some stones allow X-rays to pass through them, while others absorb them to some extent, different degrees of transparency thus being shown.
Diamond is highly transparent to the X-rays, phenakite, amber and jet also being completely transparent. Glass and pastes are
opaque, so diamond could be easily distinguished by this test. Corundum (ruby and sapphire) is nearly transparent, a property that would also distinguish it from other similarly colored stones. Opal and chrysoberyl are less transparent to these rays than is corundum; all varieties of quartz, feldspar, topaz, and spodumene are semi-transparent. Apatite, peridot, turquoise, sphene, tourmaline, beryl, and epidote are nearly opaque, feldspar transmitting light slightly. Almandine garnet, zircon, and all forms of glass are quite opaque.
We have seen that a few stones appear phosphorescent when heated, and fluorescence is a closely connected property. A fluorescent mineral has the power of making an ultra-violet spectrum visible. When this is seen, the atoms have been raised to a higher state by the absorption of light, and on returning to their normal state by stages, they emit light. This glow of light, called fluorescence, differs only from phosphorescence in as much as the latter phenomenon is seen in certain stones under the influence of ordinary light rays only, the effect being seen after such stimulation.

In a later chapter, we will note the use of other instruments to detect some of the properties of stones. Many of these instruments are expensive and require much skill and practice in use; they are therefore almost confined to laboratory work. But enough has been noted here of the various properties of gem stones to enable the reader to use two or three methods which should, in conjunction with one another, distinguish any jewel stones with which one would normally come in contact. Reference should be made to the tables in the Appendix.
The tests for a rough stone would be the observation of its crystalline form, hardness, specific gravity, and color. As many
different types of tests should be made as possible if any doubt exists. Color alone is no sure test, although constant handling of stones will give one a good idea as to what a given specimen may be from this property alone. Reputable gem dealers do not need to test stones when buying from each other. The description of a given stone is generally accepted without question.
But the buying of second-hand jewelry requires more knowledge for, in many instances, stones are not what they seem to be. During the course of time, certain stones may have been lost from their settings and replacements made which, although of the same color, may be pastes or synthetics. To the uninitiated, a white zircon may appear to be the same as a diamond, or a garnet may be easily mistaken for a ruby. Since natural sapphires are often dark in color, the synthetic sometimes presents a problem. The internal flaws of a ruby are more easily seen on account of its lighter color; moreover a stone weighing as little as three carats would always cause careful consideration. The commercial values of these stones differ vastly, so the reader will be advised to make the necessary tests we have described in order to differentiate between such stones so that no mistake can be made.

Another optical property which gem stones possess is that of luster, and it is essential for a gem stone to possess this quality to a high degree in order to make it attractive. Luster differs both in intensity and in kind, and it depends upon the amount of light reflected as well as upon the nature or quality of the reflected light. Generally, those stones which are very hard and possess a high-refractive index show the highest luster, but careful cutting and polishing is a great aid. As much light as possible should be reflected from the front of the cut stone. The different kinds of luster may be classed as follows:
Adamantine, e.g. diamond,
Vitreous, e.g. glass, quartz,
Resinous, e.g. opal, amber, and some garnets.
Waxy, e.g. turquoise,
Pearly, e.g. moonstone, pearl,
Silky, e.g. crocidolite, satin spar, and other minerals having
a fibrous structure, Metallic, e.g. gold and most metals.
Some stones exhibit certain special reflection effects, such as sheen or a play of color. These are generally due to peculiar structure, which may also cause interference of light.

Iridescence is a display of colors due to refraction and inter ference of light rays in minute fissures of stones, such fissures or small cracks resulting in thin films of air or liquid being intro duced. They are sometimes the result of incipient fracture. Iris quartz, fluor spar, and mica show this property.

A play of colors is seen in some stones, the effect being that different colors appear on the surface of the stone in rapid succession as it is moved. The cause of this is not always the same. With opal, the cause is probably the numerous minute cracks within the stone being filled with secondary opal of slightly different refractive indices from the original matter. Interference by reflection results in the blue, green, and red colors so often seen in both the black and white opal. The effect is sometimes called opalescence.
With labradorite, a plagioclase feldspar, which is dull gray in the rough state, brilliant patches of color are seen if viewed in different directions when the stone is polished or cut. This effect is caused by the structure of the stone, which consists of small, thin parallel plates (repeated twinning). Interference of light by reflection gives the play of color. Platy inclusions, which are not always present in labradorite, would give the same result.

Some stones change their color when subjected to X-rays, but tins change is only temporary. Strong sunlight or heat will result in a reversion to the original color. Diamond and kunzite both become highly phosphorescent if subjected to the action of radium emanations. Some interesting experiments have been car-tied out in this connection, and the use of X-rays in examining the internal structure of minerals is of growing importance. Much work of this nature has been carried out in recent years, and the names of von Laue, von Knipping, and Bragg are associated with such investigations. Today, the physicist regards the X-rays as one of  his most powerful accessories.
As has already been noted, the X-rays are a form of light of a particular wave length. Light waves, whether the sun or an artificial light be the source, have a narrow range of magnitude, the longest being about one thirty-thousandth of an inch and the Shortest about half as long. So light waves sweep over molecules smaller than themselves and no effect is carried to the eye or brain. A microscope is of no use to enable us to see things which are of the same size as the wave length of light. But the X-rays  are considerably finer than ordinary light, and by their use the atoms and molecules which make up crystals may be examined.
Von Laue passed X-ray radiations through crystals and photographed the emerging rays on to a sensitive photographic plate, thus obtaining on it what is now called “Laue figures.”

We have seen that solar light, light from the sun, is split up into its component parts when it passes through two inclined faces of a prism, producing the colored band known as the spectrum. This breaking up of white light is known as dispersion, and this property varies with different stones. It determines the “fire” of a stone, and a brilliant example is the diamond, whose flashing display of prismatic hues is so characteristic. A high refractive index assists this effect, for not only are the component rays of light widely separated when they enter a diamond, but the emergent rays are long and the separated rays are consequently increased.
The color of a stone may mask its “fire.” Actually, demantoid garnet shows this property more strongly than any other stone, but being green, it is not so obvious. Sphene, diamond, zircon, and hessonite garnet follow demantoid in the order given. It should be noted, however, that stones must be properly cut to show this power to the fullest advantage. The amount of dispersion may be measured for each stone, and the difference between the extreme refractive indices of the red and violet rays at the ends of the visible spectrum will give the required figure. In practice, the so-called B and G lines of certain known wave lengths are generally taken as standards.

Now if white light is passed through a stone and examined with the spectroscope, the spectrum will be crossed by a number of black lines. This effect is called an absorption spectrum, and the lines correspond to the rays absorbed. Ruby and sapphire give blurred patches, almandine garnet shows a band in the yellow part of the spectrum, and alexandrite shows blurred black bands which occupy the yellow and violet areas, the colors seen being mainly red and green.
Sharp, well defined absorption bands are seen with zircon, the spectrum being crossed by a number of clear, black bands. The cause is the presence of hafnium. These bands in the red, yellow, and blue parts of the spectrum are sufficient alone to distinguish zircon from all other stones.
The scientists Dolter and de Gramont specialized in research work connected with the color of gem stones, and they made great use of the spectrograph. This instrument can determine the wave lengths of light rays emitted by bodies which are in a state of dissociation under high temperatures. The radiations are allowed to fall on to a photographic plate, and from the nature and the number of the sharp lines so produced, the presence of the smallest quantity of matter within the stone may be ascertained. For the actual coloring agent is usually present only in a very minute guantity. Such experiments, however, need considerable experience in order to get useful results.

The two different shades of color which some stones show when viewed from different angles is known as dichroism. If three colors are seen, the stone is said to be trichroic. The different colors follow the crystal axes of the stone, so it is obvious that all crystals of the cubic system will show no dichroism. Glass and other artificial imitations will also not show this property (unless anomalously), nor will colorless stones. Garnets and synthetic spinels under strain occasionally show dichroism. Garnets, for instance, may be distinguished from red tourmalines or rubies by this property alone. But not many stones exhibit dichroism, as reference to the Appendix at the end of this book will show.

To ascertain whether a stone is dichroic, a small instrument known as the dichroscope is used. In its simple form, it consists of a hollow tube, one end of which is closed except for a slit in the cap. In the other end, another tube is sometimes fitted, and this slides so that it may be focused. There is a lens at the end of this tube. Inside the tube is a rhomb of calcite (carbonate of lime); this mineral is used because the double refraction of the two rays is very marked, and consequently two images of anything viewed will appear to be very wide apart.
When looking through the instrument, two images of the slit will be observed, but when a dichroic stone is examined and the instrument or the stone carefully rotated, there will be four positions at 90 ° to each other in which the color will appear the same in the slit. But on rotating into a different position, the color will gradually change, increasing and then decreasing in density. Some stones are so feebly dichroic that the difference in tints is difficult to observe. Yet the manifestation of any dichroism will prove the stone to be doubly-refractive. When viewed along the optic axes, no dichroism is seen and therefore the stone, or instrument, must be rotated. Stones that are dichroic are listed in the Appendix.

The dichroscope is quite a small and cheap instrument, but (here are one or two with elaborate refinements which facilitate handling. An example is the Rayner dichroscope. It has a metal base and can be used on a table; the stone can be conveniently viewed on a small platform;  the tube can be rotated and the height adjusted. But the essential principle in the construction remains the same as in other types.

Another instrument, known as the polariscope, is sometimes used for gem testing. This contains so-called Nicol’s prisms cut from calcite, which possess the property of polarizing light. Polarization results from the extinction of a certain set of vibrations in a ray of light. By its use, singly refractive stones appear black when the stage is rotated, and doubly refractive stones give four positions of light at positions of 900 to each other. Some stones, such as diamond and also some glasses, show anomalous double refraction, but in these instances there are not four distinct posi tions in which light is totally extinguished. The light is seen in patches and irregularly. This instrument is more fully explained in a later chapter.

The spectroscope and the spectrometer will also show certain results which give some indication of the nature of a stone. But these instruments, of which there are many types according to the purposes for which they are required, are almost exclusively used in the laboratory. The spectroscope is really an instrument which sorts out the waves and places them in positions corresponding to their size, the short waves at one end of the scale and the long waves at the other. Each set of waves produces its own line, but to read this “wave-meter” needs considerable experience.

The principle of the spectroscope is fairly simple. The con trivance is made up of several prisms, the edges of which alternate and which are set in opposite directions. The prisms are of crown glass and flint glass. Light passes through a slit in one end of the brass tube which contains the prisms, and then passes through the flint and crown glasses. The brightest rays in the spectrum pass through without deviation, but the red and yellow are deviated. If a light is placed near the slit and the eye applied to the eye piece, a beautiful spectrum is seen. If a piece of ruby colored glass is placed between the light and the slit, only red light is trans mitted to the eye; all other light is absorbed by the glass. Other minerals stop definite rays only.

Spectrum analysis is of great use to the chemist, since a very small quantity of any mineral will give a spectrum consisting of lines or bands characteristic of the atoms of which it is composed, The positions of such lines can be compared with the lines of  known elements, and the composition can thus be quickly and accurately   ascertained.   Kirchhoff  and   Bunsen   developed   this
branch of science in 1860 and onwards, and the name of Angstrom is connected with later work, which is still being continued.