Pleochroism

As explained before, different colors can often be observed in double refracting gems by looking at them from differ ent directions. Any color differences seen must be remembered. This dichroism or pleochroism can be seen, and the colors compared directly, without the necessity of relying on memory, by using a dichroscope. In its simplest form, this instrument uses two small squares of Polaroid sheet which are fastened with their edges touching but with their polarizing directions set at right angles to each other. The gem is viewed through the Polaroid against a strong light. If dichroism is present, the color of the gem portion seen through one section of the Polaroid will differ from that portion seen through the other. By turning the gem in several directions, a third color may possibly appear. If there are observable color differences, the gemstone is doubly refracting and cannot be an isometric mineral or glass or plastic. If only two colors are visible, very likely the mineral is tetragonal or hexagonal. Three colors will almost guarantee that the mineral is orthorhombic, monoclinic, or tri-clinic and three stone wedding rings.
Sometimes dichroism and pleochroism are a little difficult to see because the color differences may be subtle enough to escape visual detection. By itself, the use of the dichroscope is not a positive means of identifying gemstones, but it does add helpful information to that obtained by other tests.

Cleavage

Gemstones do break. Some come apart with relative ease, while others require the shock of an energetic blow or the unusual stresses that result from sudden and extreme cooling or heating. The ease with which they break up depends upon the strengths of the bonds between atoms holding them together. If the breakage is irregular, leaving uneven or jagged surfaces at the break, it is called "fracture." At times, with different minerals, the breakage will follow definite directions. The crystal structure splits where the bonding is weakest between certain planes of atoms in the internal arrangement. This causes the broken surfaces to be flat faces parallel to planes of atoms which are parallel in turn to possible crystal faces. Topaz, for example, always breaks in a manner showing flat surfaces or cleavage planes that develop in one same direction through the structure. Other minerals may exhibit cleavage in two, three, or even four different directions through the crystal structure. Feldspar cleaves in two directions which are at right angles to each other. Spodumene also has a two-directional cleavage, but the two are not quite at right angles. Since the cleavages of different kinds of gems vary considerably, they can be used to help identify the jewelry stones.

Specific Gravity

Most gems and gemstone samples are large enough to be weighed accurately, so that one of the weight-measurement methods is normally used to find the specific gravity. You will remember that specific gravity is the weight of the gem compared with the weight of an equal volume of water. The Greek mathematician Archimedes is credited with having discovered the method of determining specific gravity in the third century B.C. He realized that an object would weigh less when submerged in water and the weight loss would be equal to the weight of the water whose place was taken up by the object. In other words, the weight loss represented the weight of a volume of water equal to the volume of the object. All that is necessary to determine specific gravity is to weigh the gem accurately in air and weigh it again while it is immersed in water. A simple calculation gives the answer. The gem's weight in water is subtracted from its weight in air. This gives the weight of a volume of water equal to the volume of the gem. This weight is divided into the weight of the gem in air to find how many times it exceeds the weight of the water—thus its specific gravity.
Of course, the more accurate the scale for weighing the more precise the determination of specific gravity. For larger gems and gem-stone fragments and princess diamond earrings, cruder and less sensitive weighing devices—even homemade—are accurate enough.
As with refractive index, it is also possible to find the specific gravity of a gem or fragments of a gemstone by using a series of liquids. It is appropriately called the "sink-float" method. In this procedure the liquids have a known specific gravity. Very simply, if a stone is denser than a liquid it will sink, if less dense it will float. A more precise measurement can be obtained by first floating the gem on a denser liquid. Slowly, drop by drop, with stirring, a less dense liquid is added. Eventually, the gem will start to sink as the mixture reaches a density just slightly less than its own. Quickly, before evaporation can cause changes, the density of the liquid mix is determined; it will be equal to the density of the gem. Of course, this requires the availability of a floating instrument called a hydrometer. This floats in the liquid and the density can be taken from a number scale which is read at the mark where it settles at the surface. There are also other, more accurate and more expensive kinds of weighing devices for finding the density of such mixed liquids.

Chatoyancy and Asterism

The word "chatoyancy" comes from a contraction of the French words chat for cat and oil for eye; it aptly describes this odd reflection phenomenon. The cat's-eye effect is a sharp single band of light running like a brilliant slit across an oval-shaped stone. It looks for all the world like a glowing cat's eye. The light band is multiple reflection from thousands of needlelike inclusions in the gemstone, all running parallel to each other. Interestingly, the thinner and more numerous the inclusions, the sharper and brighter the eye. These needlelike structures may be the mineral species rutile, or may even be extremely thin, empty, capillary tubes. Although the finest cat's-eyes occur in chrysoberyl, they have also been found in some tourmaline, ruby, sapphire, garnet, spinel, and even quartz.
If, by chance, the needle-like inclusions are lined up in two or even three directions related to the crystal structure, the chatoyancy becomes more complex and two or three light bands are reflected. Thus the very popular rubies and sapphires showing asterism, or a star effect, are merely displaying their chatoyancy in three directions with the bands of light intersecting at a single point. Of course, it is necessary to cut such a stone carefully in the proper crystal direction, so that the intersection of the light bands falls at the center of the peak of the rounded gem. Unfortunately, if the inclusions are lacking or more sparse in one crystal direction than in another, a star with weak or missing legs results. All star and cat's-eye stones perform better if viewed under a single point source of light, such as the sun or an incandescent light bulb. Other light sources are likely to be so diffuse as to produce only diffuse reflections.

Other Physical Characteristics

A gem's effect on light provides many of its attractive features. Other kinds of behavior it may exhibit are just as important and interesting. They, too, are the direct result of the chemical composition and atomic structure of the gemstone species. Among these characteristics are hardness, cleavage, density, and certain electrical properties. For most gemstones, a few of these characteristics—such as hardness, density, and refractive index—are sufficient to make positive identifications. That goes also for 3 stone rings.

Hardness

One of the best ways to think of hardness is as the scratch-resisting ability of the gem. Hardness is directly related to the tenacity of atomic attractions. There are great differences in the strengths of the bonds by which different kinds of atoms are held together. Naturally, some combinations will resist being torn apart more than others. The atom bonding in diamond jewelry is so very strong that the species is exceptionally hard and cannot be scratched or torn apart by other substances. Softer gems, such as amethyst, are much less strongly bonded and are soft enough so that repeated exposure to scratching forces will leave them badly marked. Since the early 1800's a rough but convenient scale for measuring hardness, originated by the German mineralogist Friedrich Mohs, has been in general use. The scale is based on ten relatively common minerals ranked from 1 to 10 in the order of increasing hardness: 1) talc, 2) gypsum, 3) calcite, 4) fluorite, 5) apatite, 6) feldspar, 7) quartz, 8) topaz, 9) corundum, 10) diamond. The degree to which hardness increases between the numbers is not at all uniform. There is a greater difference between the hardness of corundum and diamond—9 and 10 in the scale—than between numbers 1 and 9. Almost all important gemstones have a hardness above 6 in this scale. Anything less than 6 is not durable enough to resist the scratching and chipping of general use. For practical purposes it is useful to know that window glass is usually slightly softer than 6, a good steel knife is 6 to 61/2, and a hard file is close to 7. One of the odd variations in the hardness of some gemstones occurs when there is an appreciable difference in the strengths of the atomic bonds in different directions through the structure. Kyanite, for example, varies from 5 to 7 in hardness, depending on the direction in which an attempt is made to scratch it.

Luminescence

Most of the light effects discussed so far result from reactions between gemstones and visible light. However, certain gemstones do react to the stimulus of radiation which is beyond the limits of visible light. The most striking effects come from subjecting gemstones to short wavelengths just beyond the violet limits of the light spectrum. This ultraviolet light, or the even shorter X-rays, is absorbed and then given off again as longer and often visible wavelengths, according to a law discovered by Sir George G. Stokes in 1852. Thus, ruby when exposed to an ultraviolet light source will glow like a dull red coal, and some diamond rings may assume an eerie, bluish luminescence. The phenomenon is named "fluorescence" after the mineral fluorite in which it was first studied.
Occasionally, the re-emission of this changed shortwave radiation is delayed by the mineral. This phenomenon is known as "phosphorescence." The famous Hope Diamond, when exposed to strong ultraviolet radiation, produces little fluorescent reaction. However, it is startling to see the stone, once the exciting shortwave radiation is removed, glowing with a brilliant scarlet, delayed phosphorescence.
Luminescent effects in gemstones are not particularly important except as identification aids. Unfortunately, a milky blue fluorescence, when it occurs in some diamonds used for diamond bridal engagement rings under sunlight or strong incandescent light, can detract considerably from brilliance and value.

Chatoyancy and Asterism

The word "chatoyancy" comes from a contraction of the French words chat for cat and oeil for eye; it aptly describes this odd reflection phenomenon. The cat's-eye effect is a sharp single band of light running like a brilliant slit across an oval-shaped stone. It looks for all the world like a glowing cat's eye. The light band is multiple reflection from thousands of needlelike inclusions in the gemstone, all running parallel to each other. Interestingly, the thinner and more numerous the inclusions, the sharper and brighter the eye. These needlelike structures may be the mineral species rutile, or may even be extremely thin, empty, capillary tubes. Although the finest cat's-eyes occur in chrysoberyl, they have also been found in some tourmaline, ruby, sapphire, garnet, spinel, and even quartz.
If, by chance, the needle-like inclusions are lined up in two or even three directions related to the crystal structure, the chatoyancy becomes more complex and two or three light bands are reflected. Thus the very popular rubies and sapphires showing asterism, or a star effect, are merely displaying their chatoyancy in three directions with the bands of light intersecting at a single point. Of course, it is necessary to cut such a stone carefully in the proper crystal direction, so that the intersection of the light bands falls at the center of the peak of the rounded gem. Unfortunately, if the inclusions are lacking or more sparse in one crystal direction than in another, a star with weak or missing legs results. All star and cat's-eye stones perform better if viewed under a single point source of light, such as the sun or an incandescent light bulb. Other light sources are likely to be so diffuse as to produce only diffuse reflections.

Transparency and Luster

The two light-reaction phenomena of transparency and luster can almost be thought of as direct opposites. The transparency of a gem is a description of the ease with which light travels through it, while luster is a description of the way in which light is reflected back from it. Transparency is an interesting phenomenon because, except for the evidence of our senses, it seems impossible that anything can pass through a solid substance. And then there is the further problem of explaining how some solids will let the light through and others won't. The fact is that light doesn't go through anything. What happens is that light hits the atoms at the surfaces of these solids and, by its own energy, starts them vibrating sympathetically. These vibrations are passed through the structure from atom to atom. If the atoms are properly aligned, the vibrations will move as in a row of falling dominoes. They are then ejected at the other side in the identical form in which they entered. The same goes for vintage style engagement rings.

The light does not trickle slowly through spaces between the atoms as though finding its way through a maze. The rate of travel of the vibrations is close to 670 million miles an hour, which makes its passage seem almost instantaneous.
Luster, being a reflective effect from the surface of the stone, depends on the quality and quantity of the light thrown back. This, in turn, depends on how the stone tends to reflect rather than refract and also on how well the stone can be polished. Both in turn result from the kind of internal structure it has. Most gemstones reflect like ordinary glass and their luster is described as glassy or vitreous. Some, such as zircon and garnet, have a high luster called "adamantine," or diamond -like. A convenient descriptive classification of the kinds of luster might also include resinous (or greasy), pearly, and silky.