X-Ray Examination

Although the kind of X-ray examination of most value to gemology requires the destruction of a small amount of the stone it remains the most important single key to identification. When a beam of X-rays is directed at a solid, much of it passes through the solid with no alteration, some of it is scattered, and some is converted to heat or other kinds of energy. It is the scattered X-rays that are of interest because they are the ones that have hit atoms on the way through. A picture of these scattered X-rays is taken by proper placement of a photographic plate. A large number of atoms, all uniformly spaced and placed in the structure, will scatter the X-rays in the same direction and reinforce their image on the film. The others are scattered in various directions and produce no combined mark on the film. This means that every different plane of atoms in the structure leaves its print on the film. By careful measurement of the markings on the film and suitable mathematical treatment of the measurements, the entire internal atomic structure is revealed.
The X-ray method most often used is the powder diffraction procedure. The camera is a flat hollow metal cylinder, one end of which is a removable lid. It is very carefully machined to exact dimensions, since its uniformity and the size of its diameter are crucial in the final film measurement. There is a hole on one curved side for the entrance of X-rays, and a hole opposite for the exit of most of them. Tapered metal tubes fit in these entrance and exit ports to guide the X-rays to and from the sample. The sample is mounted on a rotating spindle at the exact center of the camera. The film is a long, narrow strip that fits flat against the inside wall of the cylinder. It has two holes that fit over the entrance and exit port tubes.
In operation, the sample—a tiny bit of the mineral which has been powdered and then held together by various adhesives—is mounted, the film is loaded in the darkroom, and the lid is replaced. As the sample spindle is turned by a motor-driven belt, the entrance port of the camera is placed against the X-ray source and the exposure proceeds, taking several hours. When the film is developed, it has a series of matched curved lines running across it. These lines represent atomic planes; for each species their pattern is characteristic and will be different from that of any other species. The films can be indexed and filed and used much the same way as fingerprints.
All told, then, in their investigation and study of gemstone species and gems through the years, mineralogists and gemologists have assembled a rather impressive arsenal of instruments and techniques. The accumulation of facts has also proceeded steadily until we have reached a point where the many problems of gemstone identification and gem preparation are matched by sufficient knowledge to solve them. It is a very rare or unusual natural gem material that worries a qualified and competent gemologist. However, man is also perfecting his ability to manufacture gemstones. It is obvious then that the science of natural gem-stones and diamond engagement rings is essential if the distinctions between natural and man-made gems are not to be obscured.

Gemstone Inclusions

The student gemologist, when he first begins to look inside gem-stones under magnification is often amazed at the myriad of included objects he sees there. Tadpole, comma, round, or elliptical bubbles abound as well as fibrous horsetails, cracks, beautiful tiny crystals, blades, feathers and mossy traces of matter. At magnifications of from 10 to 40 times under the microscope it is often possible to recognize inclusions that tell not only what the gem is but where in the world it came from. Tiny actinolite blades in an emerald signal immediately that it was mined in Russia and rtot at the famous mines of Colombia.
These inclusions may have arrived in the gemstone at different times. Some existed before the gemstone was formed and they were swept up and trapped in the developing solid. Even tiny droplets of the liquid from which the crystal formed are sometimes trapped. Some liquid-filled inclusion cavities have tiny gas bubbles that move back and forth in their small prisons as the stone is tilted and other diamond jewelry. Now and then, a mineral species developing from the same liquid as the gemstone leaves its trace as a scatter of bright, little but well-formed crystals peppered through the stone. Often, too, after the gemstone has formed it develops a series of tiny cracks and fissures. These may later be filled by the infiltration of liquids which form new crystalline material to "heal" the breakage. This accounts for the typical "healed" feathers seen in Ceylon sapphires. There are certain significant internal features caused by accidents during growth. Color zoning, sometimes not too obvious without magnification, will appear as definite bands of differing color intensity due to interruptions during growth or slight changes in the content of the supply of material brought to the forming crystal. Prominent hexagonal color zoning is typical of Burmese sapphire.

Spectrum Analysis

For colored gemstones it is often possible to obtain very useful information for identification by use of the gem spectroscope. The instrument's operation is based on the separation of white light into its complete rainbow, or spectrum, of colors. This is done by a built-in prism which receives the light through a narrow slit. The prism sorts out the various wavelengths by its strong dispersion. Often a diffraction grating is used instead of a prism to diffract or separate the colors. Looking through the opposite end of the instrument one can see the continuous rainbow as a band of touching parallel bars or lines of different colors. They range from violet and indigo at one end, through blue, green, yellow, orange, to red at the other end. Now the colored gem is placed between the light source and the spectroscope slit. As expected, certain specific colors from the white light are absorbed by the gem and do not enter the spectroscope. The result can be seen in various white gold engagement rings. Their absence causes black bars—the absence of specific colors—to appear in the continuous spectrum of color bars. For some colored gems these patterns of black bars are very distinctive and are good identification features.

Finding Gem Characteristics

The gemologist often is handicapped in trying to determine gem characteristics. If the stone is mounted in some kind of jewelry setting, manipulation for tests is difficult. Owners frequently object to removing stones from their mountings since damage is possible. Even if the stone is not mounted, it is not practical to risk ruining the cut or polish by scratching, chipping, or removing a piece for testing. Every bit of damage reduces the value of the gem. Testing on cut stones is usually limited to nondestructive manipulation. Unfortunately, some of the best gem study techniques involve destruction of the sample. Destructive tests are carried out, then, on uncut gemstones. Among such tests are chemical analysis, X-ray structure determination, and Mohs' tests for hardness. Nondestructive tests include determination of refractive index, specific gravity, pleochroism, spectral pattern, and examination for any foreign inclusions in the stone.

Hardness

As already mentioned, hardness testing is destructive or damaging to the stone. It is determined by actually trying to scratch the stone with some of the minerals in Mohs' scale of hardness. Bits of the harder minerals in the scale—from 5 (apatite) to 10 (diamond)—can be obtained already mounted in small metal rods for convenient manipulation. Carefully, an attempt is made to scratch the gem with one of these hardness pencils, perhaps #7. The scratching is done under magnification and along the edge of the gem where it will not mar any of the facets. Only a tiny, almost invisible scratch is necessary. If #7 will not produce a scratch, #8 is tried. Should #8 produce a scratch then it is obvious that the gem's hardness lies between $1 and #8. Estimates can be made about whether it is 714, 71/2, or 7%, depending on how easily #8 made the scratch.

Refractive Index

It has already been explained how minerals refract or bend a beam of light. As the light hits the flat surface at an angle, it bends upon entering the gem. If the light beam's direction is slowly changed so that it comes to the surface of the gem at an increasingly lower angle, eventually a point is reached where it ceases to bend sufficiently to enter the gem. It just grazes the surface. Any further lowering of the beam causes it to be totally reflected away from the gem. The grazing angle is called the critical angle, and it will differ with each gem substance according to its refracting ability.

An instrument, the gem refractometer, has been devised to measure this critical angle quickly and easily. The instrument usually contains a built-in scale from which the refracting ability, or "refractive index," of the gem can be read directly. To read the index on a typical gem refractometer, of which there are several models on the market, one of the polished faces of the gem is placed against a polished piece of very highly refracting glass mounted in the instrument. Good contact is assured by placing a drop of highly refracting liquid between them. A light beam is brought through the glass to the gem. Any of the light coming to the gem from an angle at which it will be refracted is bent into the gem, away from the instrument, and is lost. Light coming in at an angle beyond the critical is reflected back into the instrument, hits the viewing eyepiece, and its trace shows as a bright section on the scale. The numerical marking on the scale dividing the light portion—representing reflected light—and the dark portion—representing the lost light refracted into the gem—is the critical angle. For convenience, this is numbered on the scale as the refractive index. The measurement is sufficiently precise so that, by consulting a table listing the refractive indices of gemstones, one can usually make a quick identification of the gem in question.

There are other methods for determining refractive index that are somewhat less convenient. One of the best of these uses a series of fluids of known refractive indices. When a gem is placed in a liquid having the same refractive index it effectively disappears. This happens because the gem does not cause any further bending of light than that already done by the liquid. We have no visual evidence of the gem's presence because it behaves toward light exactly as the liquid does. And also round diamond earrings An appropriate series of test liquids such as clove oil (index 1.54) to test for quartz (index 1.54), cassia oil (index 1.60) to test for topaz (index 1.61) and methylene iodide (index 1.74) to test for spinel (index 1.72-1.73) is easily assembled. Although some of the liquids are expensive and difficult to obtain and may be unstable enough to need frequent replacement, the method has some benefits for example diamond solitare earrings. It can be used with very small gem-stones fragments, and the flat, polished surfaces required for the refractometer are not necessary. Also, even when the gem material disappears in a liquid of appropriate index, the cracks, flaws, and foreign inclusions in it do not vanish. This affords an excellent method of checking a gemstone internally to see how it can best be cut to make the most perfect gem. It also may make it easier to see and study the nature of the inclusions.

Electrical Properties

Gems may have a number of other interesting characteristics unrelated to their appearance or durability. Among these, electrical behavior is sometimes remarkable. Museum curators and jewelers have long known that their tourmaline specimens and gems will accumulate thick coats of fine dust in a short period of time even when they are displayed in tightly sealed showcases. As the case lights are turned on and off each day the tourmaline is alternately heated and cooled. When heated, tourmaline develops a substantial electric charge which quickly attracts tiny dust particles in the air. Before long, a gem will be coated. This characteristic is known as pyroelectricity—electricity produced by heating. Diamond, topaz, tourmaline, and amber, when polished briskly with a cloth, will even develop enough of an electric charge to attract and hold small bits of paper.
An electrical effect discovered by Pierre and Jacques Curie in France in 1880 is perhaps even more remarkable. These two physicists, studying the ability of crystals to conduct electricity, found that when certain crystals were squeezed they developed a measurable electrical charge. The phenomenon was later called "piezoelectricity," based on a Greek word piezin, meaning "to press." As early as 1881 another Frenchman, G. Lippman, predicted that a reverse phenomenon would take place. He suggested that an electric charge placed on any piezoelectric crystal would cause it to change shape. This was successfully tested by the Curie brothers. Today this important characteristic is applied, using quartz and other piezoelectric substances, to control the frequencies of all radio broadcasting and other electronic devices. The alternating current in these devices and the piezoelectric crystal plates inserted in them must operate in unison. Thus, the fixed dimensions and structure of the plate keep the current alternating only at the rate at which the crystal plate can change its shape. Such a device is known as a frequency control oscillator. It is responsible for the fact that every time you tune in your favorite radio or television station it is at the same number on the dial, just where it was last time.

Weight and Specific Gravity

: Contrary to popular understanding, the fact that a gem weighs, say, ten carats has little to do with its size, except when compared with a gem, 1 carat diamonds or 2 carat diamonds of its own species and of similar cut. The carat is an expression of weight, not size. One carat is equal to one-fifth of a gram; in more familiar units, there are about 140 carats in an ounce. This standard unit of measurement of gem-stone weights was not legalized in the United States until 1913. Before 1900 the several weight units used throughout the world differed somewhat, but the carat in one form or another had been used since ancient times. Speculation relates the weight of the carat to the weight of a seed of the tropical carob tree. These seeds are unusually constant in weight and are just slightly lighter than our legal metric carat. The ancient Greek weight—the ceratium—and the Roman siliqua are both about the same as the carob seed. Most likely, then, the name and the weight did originate with the carob tree.

Even knowing that the carat is an expression of weight only, gem buyers are often surprised to see that a one-carat sapphire is considerably smaller than a one-carat diamond. Sapphire is denser than diamond and a smaller stone can weigh the same number of carats as a larger diamond. Since size and weight are both very important measures for gemstones, the unit of .measurement that combines both— "density"—is fundamental.

It is complicated to think of a gem's density, because size and weight must be considered simultaneously. We are aware of a difference in weight when we compare iron and wood. Yet it would not always be correct to say that iron weighs more than wood because a large piece of wood can weigh more than a small piece of iron. Only by comparing equal volumes of these materials can the extent of the weight difference be made clear and unmistakable. Making innumerable comparisons of the weights and volumes of different solids with each other seems inconvenient. To avoid this, it is customary to compare all of them with some selected substance, readily available, with a known density. Water is the standard most often used. Ruby, for example, proves to be four times heavier than an equal volume of water, so its specific gravity—the term of comparison with water—is 4. Diamond weighs SlA times as much as an equal volume of water and thus has a specific gravity of 3Vi.

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.