Although it resembles interference color, the peacock play of colors in opal, which is also pseudochromatic, arises from still another process—one which has not been understood until recently, when it became possible to take electron microscope photographs of up to 40,000 magnifications. In these photographs, precious opal is seen to consist of layer upon layer of silica spheres (Si02) arranged row upon row in neat, orderly grid patterns with relatively uniform spacings between the spheres.
This arrangement acts like the optical-laboratory device called a diffraction grating. This is usually made by scratching a series of fine parallel lines on a glass or metal plate with a diamond point. The lines are spaced as many as 30,000 to an inch. Portions of a light beam directed at such a grating are reflected back from each of the thousands of polished gaps between the scratched lines. Using just one of these tiny "beamlets" as an example, the part that is closest to the edge of a neighboring scratch is bent from its expected path. This bent or diffracted portion is now thrown out of phase with the rest of the beam-let and is in a position to cause interference with its neighboring light waves. Also check princess diamond earrings
The behavior of this single tiny reflection is repeated by all the thousands of others, giving a uniform interference color all across the grating. Precious opal shows its diffraction colors in patches. This is because the grating-like arrangement of silica spheres occurs in irregular patches and the patches are not necessarily oriented in the same direction. The thickness and spacing of the scratched lines of a diffraction grating have a direct effect on the interference colors produced. The relative positions of the light source, the grating, and the observer also help to determine the colors. So it is with viewing opal. The size and spacing of the silica spheres and the relative positions of the light source, the opal, and the observer make striking differences in the pseudochromatic colors seen.
Showing posts with label jewelry. Show all posts
Showing posts with label jewelry. Show all posts
Thursday, July 10, 2008
Thursday, July 3, 2008
Internal Structure of Gems
Internal Structure: The particular combining abilities of each kind of atom go a long way toward determining what combinations or compounds are possible. At the time a mineral forms, there are restrictions relating to the size, characteristics, and numbers of atoms present. Atoms are energetic, and exhibit this as rapid, erratic motion. As they rush about at phenomenal speeds they tend to fasten onto each other by strong attractive forces. Many trillions of atoms may pack themselves together this way in the course of an hour during the formation of one of these mineral solids.
This would suggest that they all end up in a great, unstable, chaotic mass. Instead, because of the uniform distribution of attractive forces and relatively uniform sizes, they line up in remarkably orderly, repetitious, geometric patterns and hold themselves quite tenaciously in these patterns called crystal lattices. A good demonstration of how this happens can be prepared by shaking up a basketful of tennis balls. They all quickly settle down into an orderly geometric stacking pattern as they come to rest against each other. Nature permits surprisingly few stacking patterns, and all solid mineral crystals prove to have their atoms arranged in one of fourteen basic patterns, or combinations of these patterns. In any such pattern a foreign atom or impurity atom would have to have nearly the same size and attractive power as the others in order to fit into the structure. Atoms too large or too small are rejected and cannot enter the combination. It is not unusual to see iron atoms substituting for manganese atoms in some structures and chromium substituting for aluminum in others. Each member of the pair is quite close to the other in size and attracting ability and is, therefore, not rejected by the structure.
This would suggest that they all end up in a great, unstable, chaotic mass. Instead, because of the uniform distribution of attractive forces and relatively uniform sizes, they line up in remarkably orderly, repetitious, geometric patterns and hold themselves quite tenaciously in these patterns called crystal lattices. A good demonstration of how this happens can be prepared by shaking up a basketful of tennis balls. They all quickly settle down into an orderly geometric stacking pattern as they come to rest against each other. Nature permits surprisingly few stacking patterns, and all solid mineral crystals prove to have their atoms arranged in one of fourteen basic patterns, or combinations of these patterns. In any such pattern a foreign atom or impurity atom would have to have nearly the same size and attractive power as the others in order to fit into the structure. Atoms too large or too small are rejected and cannot enter the combination. It is not unusual to see iron atoms substituting for manganese atoms in some structures and chromium substituting for aluminum in others. Each member of the pair is quite close to the other in size and attracting ability and is, therefore, not rejected by the structure.
Monday, June 30, 2008
Interference
Some gemstones, such as moonstone, are "pseudochromatic." That is, of growth has caused the development of various kinds of films or layers. Interference colors are the usual result. In describing the process of interference, the usual procedure is to invoke an image of the rainbow play of colors on a thin oil slick on a rain-wet street.
A ray of light strikes the thin layer of oil at some angle. Some of the ray is reflected immediately from the top surface of the oil; some penetrates the thin film, and, in turn, is reflected from the contact surface where the oil film rests on the water. This second ray portion, traveling back through the oil film, continues on its way parallel to the first ray fraction.
However, it is retarded because it has traveled a slightly longer distance. This means that the light waves in the two parts bouncing back have gotten out of step with each other. Since light waves are additive, the resulting combination of out-of-step portions in the eye of the observer is of a different mixture of wavelengths from the original ray or, by definition, a different color blend. The hue produced by these interfering wavelengths depends on the thickness of the film and the angle at which the ray of light strikes it. If the film is too thick or too thin, interference effects are lost. Moonstone offers a good example of the "schiller," or glow of color produced by interference effects. It contains very thin layers of the mineral albite alternating with very thin layers of the mineral orthoclase. These layers act as films, thus producing the popular bluish and ghostly internal glow by interference when struck by a ray of light. The play of iridescence or tarnish colors on some metals is interference color due to the formation of very thin films of various oxides or sulfides left on the metals by the chemical attack of gases or solutions.
A ray of light strikes the thin layer of oil at some angle. Some of the ray is reflected immediately from the top surface of the oil; some penetrates the thin film, and, in turn, is reflected from the contact surface where the oil film rests on the water. This second ray portion, traveling back through the oil film, continues on its way parallel to the first ray fraction.
However, it is retarded because it has traveled a slightly longer distance. This means that the light waves in the two parts bouncing back have gotten out of step with each other. Since light waves are additive, the resulting combination of out-of-step portions in the eye of the observer is of a different mixture of wavelengths from the original ray or, by definition, a different color blend. The hue produced by these interfering wavelengths depends on the thickness of the film and the angle at which the ray of light strikes it. If the film is too thick or too thin, interference effects are lost. Moonstone offers a good example of the "schiller," or glow of color produced by interference effects. It contains very thin layers of the mineral albite alternating with very thin layers of the mineral orthoclase. These layers act as films, thus producing the popular bluish and ghostly internal glow by interference when struck by a ray of light. The play of iridescence or tarnish colors on some metals is interference color due to the formation of very thin films of various oxides or sulfides left on the metals by the chemical attack of gases or solutions.
Gemstones - Just Minerals Found In Earth?
Since gemstones, with a few notable exceptions, are minerals found in the earth's crust, the laws and procedures applied to the study of minerals fit them perfectly. Any trained mineralogist can soon become a competent gemologist, since he is already familiar with the techniques of identification and knows the fundamental chemistry and physics of natural substances. A mineral is a natural substance having a definite chemical composition and definite physical characteristics by which it can be recognized and distinguished from other substances. Technically, in mineralogy, those natural substances formed by living organisms are excluded. This means that amber and jet, formed by plants, and coral and pearl, produced by animals, are not minerals. However, all four are traditionally included among the gemstones, because they qualify on grounds of beauty, rarity, etc.
The gem mineral's characteristics of brilliance, beauty, and durability arise directly from the kind of chemical composition and also from the kind of internal atomic structure it has. Sometimes, natural accidents of growth and the introduction of impurities during the formation of the gem minerals may enhance their interest and value. On the other hand, severe accidents of growth may destroy their usefulness. Obviously, some understanding of the chemical and physical reasons for mineral characteristics is needed to appreciate and understand gemstones.
Chemical Composition: We know that the universe is made up of a relatively few basic building materials, the hundred-odd chemical elements. Some of their names—gold, silver, copper, sulfur, and oxygen—are very familiar. Popular are especially gold diamond engagement rings Others such as beryllium, zirconium, and boron sound less familiar but are important among gem minerals. Still others are so rare as to be of no importance or interest in this discussion. A small number, perhaps twenty-five, supply materials to make up all significant gemstones. A few more, present in tiny trace amounts, may impart color or other occasional special characteristics.
The elements which go into making up a mineral exist as innumerable, extremely small bodies called atoms. Each kind of atom— e.g., silicon or oxygen—has its own characteristic size and its own particular ability to join with other atoms. In nature, under various temperatures and pressures and in different mixtures, the elements are brought together and combine with each other to form minerals. Since 46y2 percent of the earth's crust is oxygen and 27i/2 percent is silicon, it is not surprising that most minerals contain these two elements.
The gem mineral's characteristics of brilliance, beauty, and durability arise directly from the kind of chemical composition and also from the kind of internal atomic structure it has. Sometimes, natural accidents of growth and the introduction of impurities during the formation of the gem minerals may enhance their interest and value. On the other hand, severe accidents of growth may destroy their usefulness. Obviously, some understanding of the chemical and physical reasons for mineral characteristics is needed to appreciate and understand gemstones.
Chemical Composition: We know that the universe is made up of a relatively few basic building materials, the hundred-odd chemical elements. Some of their names—gold, silver, copper, sulfur, and oxygen—are very familiar. Popular are especially gold diamond engagement rings Others such as beryllium, zirconium, and boron sound less familiar but are important among gem minerals. Still others are so rare as to be of no importance or interest in this discussion. A small number, perhaps twenty-five, supply materials to make up all significant gemstones. A few more, present in tiny trace amounts, may impart color or other occasional special characteristics.
The elements which go into making up a mineral exist as innumerable, extremely small bodies called atoms. Each kind of atom— e.g., silicon or oxygen—has its own characteristic size and its own particular ability to join with other atoms. In nature, under various temperatures and pressures and in different mixtures, the elements are brought together and combine with each other to form minerals. Since 46y2 percent of the earth's crust is oxygen and 27i/2 percent is silicon, it is not surprising that most minerals contain these two elements.
A Good Diamond
A good diamond with its high degree of brilliance and fire, as well as extreme hardness and rarity, comes close to the ideal. Opal, being relatively fragile and having little but its rarity and breathtakingly beautiful play of colors to offer, still qualifies. A number of gemstone species, such as the beautiful blue Tanzanite, have produced beautiful cut gems, but their commercial success has been sharply limited by insufficient supply—or rarity carried to an extreme. With certain notable, fashionable exceptions, a gemstone can't afford to be too rare. Scarcity does not make a stone less important as potential gem material. It merely points up the strong effect on gem marketability of the accidental, uneven, natural distribution of these species in the earth. When supply is adequate, certain attractive gems, such as spinel, still do not compete as they should with other more plentiful kinds—tourmaline, for example —that exhibit similar ornamental characteristics. Still other mineral species in adequate supply, such as fluorite and sphalerite, which are beautiful when cut and are prized by collectors, are entirely too soft, are too easily broken or cleaved, or have some other physical weakness which makes them useless as commercial gem-stones. Through a complex combination of accidents, then, certain mineral samples assume an intrinsic importance as gemstones. Continents have been explored, wars fought, crimes committed, fortunes made and lost, all in pursuit of these uncommon bits of minerals.
Gemology science
The science of gemology is i concerned with investigating and establishing facts about gems and gemstones. Somehow, because gems usually are objects of monetary value, it is often difficult to think of them in scientific terms. Questions about where they come from, what they are made of, and how they can be distinguished from one another usually take second place to questions about their value. However, these other questions must be answered first, before decisions can be made about value. For example, the market values of look-alike quartz and topaz differ greatly. Proper identification of a suspected topaz is crucial before a relative price can be assigned to it. Aside from pricing problems, other kinds of serious mistakes can be made because of faulty identification. Two of the more famous rubies in the world, the Black Prince's Ruby and the Timur Ruby, are not rubies at all, but spinel. Even so, they are extremely important stones because of their historic past and their prominence, among the British crown jewels. The science of gemology is quite different from the lapidary art, which deals with techniques for cutting, polishing, and generally shaping gemstones for ornamental use by themselves or in jewelry. Cutters and carvers and craftsmen through the centuries have refined the techniques and equipment for gem cutting. Lapidary work has become a joint venture for artist and craftsman, and it leans heavily on the science of gemology. Certainly, a lapidary must know how his gemstone is going to behave and how to turn its characteristics to advantage before he begins to work on it.
To most people who talk about gemology and about gems in general it becomes obvious that, through careless usage, some of the basic vocabulary has become confused. To set the record straight, "gemstones" are the specially treasured minerals found in the earth and "gems" are the objects fashioned from them. "Jewels" are gems that have been prepared for mounting in jewelry or other objects of art. Remember gems are used also for Wedding Bands, Wedding Jewelry
Why are only certain natural mineral samples specially treasured as gemstones? Because gems can be cut from them that have at least some of the qualifying characteristics: brilliance, beauty, durability, rarity, and portability. If the gem also happens to be "fashionable" it acquires status. Partial qualification is more often the rule, because seldom does a gem have a large measure of all these attributes.
To most people who talk about gemology and about gems in general it becomes obvious that, through careless usage, some of the basic vocabulary has become confused. To set the record straight, "gemstones" are the specially treasured minerals found in the earth and "gems" are the objects fashioned from them. "Jewels" are gems that have been prepared for mounting in jewelry or other objects of art. Remember gems are used also for Wedding Bands, Wedding Jewelry
Why are only certain natural mineral samples specially treasured as gemstones? Because gems can be cut from them that have at least some of the qualifying characteristics: brilliance, beauty, durability, rarity, and portability. If the gem also happens to be "fashionable" it acquires status. Partial qualification is more often the rule, because seldom does a gem have a large measure of all these attributes.
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