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.
Wednesday, October 1, 2008
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.
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.
Friday, September 5, 2008
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.
Monday, September 1, 2008
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.
Thursday, August 14, 2008
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.
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.
Tuesday, August 12, 2008
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.
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.
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