A mineral whose own particular composition and structure are responsible for the colors that are passed through or reflected back is said to be "idiochromatic." The gemstone peridot is idiochromatic because it is always green and must be green because of what it is, chemically and physically. On the other hand, beryl can be yellow, green, blue-green, colorless, or even pink. Absolutely pure beryl is colorless. The colors in this case arise from the presence of trace amounts of impurities.
Minerals with variable color depending on trace impurities, or sometimes on structural imperfections, are said to be "allochromatic." Both idiochromatic and allochromatic colors are caused by the interaction between light and some of the electrons that are part of the atoms that compose the mineral or are present as impurities. Atoms whose electrons commonly cause color in solids are usually of a group of elements called transition elements. These include copper, iron, manganese, chromium, nickel, cobalt, vanadium, and titanium. When these coloring agents, or "chromophores," are present in a gem the color is quite stable, but it can be changed under any conditions, such as excessive heat or radioactivity, which will change the nature of the chromophore.
Showing posts with label gems. Show all posts
Showing posts with label gems. Show all posts
Tuesday, September 30, 2008
Sunday, August 17, 2008
Crystals
Crystals: Since the internal organization of mineral solids is so orderly, there should be, and frequently is, some external evidence of this order. If the solid crystal grows without external interference it will end up with flat, shiny surfaces, or crystal faces, that are parallel to layers of atoms within. It is even possible, by measurement of the angles which these external faces make with each other, to learn something of the atomic arrangement within. This is the business of the crystallographer. Since gem material is usually of high purity and has normally formed under ideal conditions in the earth, it is often found in transparent, well-formed crystals with good external faces. Often they are so well formed with regular, shiny faces that they give the impression of having already been cut and polished as gems. In size they may vary from microscopic individuals up to single crystal monsters weighing several tons each. Good, clear, clean, gem-stone crystals, except for quartz, seldom are found in large sizes.
All solids, with minor exceptions, have orderly internal atomic arrangements and so are classed as crystals. The modern crystallographer studies these arrangements by using specialized X-ray techniques. Also, less frequently, he will study the morphology or external shape and symmetry of these crystals. The science has advanced to the point where it is possible to determine just which kinds of patterns and symmetries may occur in natural and man-made crystals. All crystals, it has been discovered, can conveniently and logically be divided into thirty-two different kinds of symmetry groups or crystal classes.
For the sake of further convenience and simplification, it is possible to group these thirty-two classes into six crystal systems, all classes in a given system having some important symmetry in common. The systems are: isometric, tetragonal, ortho-rhombic, monoclinic, triclinic, and hexagonal. The gem minerals can often be identified and distinguished from each other by the crystal systems into which they fall as they grow according to the dictates of their atomic structures. Beryl, in all its varieties such as emerald and aquamarine, is hexagonal, as is corundum with its varieties ruby and sapphire. Spinel is isometric, like diamond rings and garnet, while topaz is orthorhombic and zircon is tetragonal. Everything that a gemstone rings is, how it looks, how it wears, and how it takes cutting and polishing, depends directly on its chemical composition and its internal structure. What a gemstone's characteristics are and how they arise makes an interesting study.
All solids, with minor exceptions, have orderly internal atomic arrangements and so are classed as crystals. The modern crystallographer studies these arrangements by using specialized X-ray techniques. Also, less frequently, he will study the morphology or external shape and symmetry of these crystals. The science has advanced to the point where it is possible to determine just which kinds of patterns and symmetries may occur in natural and man-made crystals. All crystals, it has been discovered, can conveniently and logically be divided into thirty-two different kinds of symmetry groups or crystal classes.
For the sake of further convenience and simplification, it is possible to group these thirty-two classes into six crystal systems, all classes in a given system having some important symmetry in common. The systems are: isometric, tetragonal, ortho-rhombic, monoclinic, triclinic, and hexagonal. The gem minerals can often be identified and distinguished from each other by the crystal systems into which they fall as they grow according to the dictates of their atomic structures. Beryl, in all its varieties such as emerald and aquamarine, is hexagonal, as is corundum with its varieties ruby and sapphire. Spinel is isometric, like diamond rings and garnet, while topaz is orthorhombic and zircon is tetragonal. Everything that a gemstone rings is, how it looks, how it wears, and how it takes cutting and polishing, depends directly on its chemical composition and its internal structure. What a gemstone's characteristics are and how they arise makes an interesting study.
Monday, June 30, 2008
Refraction
In addition to sifting and sorting and mixing colors or light wavelengths, solid crystal structures can perform other marvelous operations with light. They are even capable of bending it or, perhaps more correctly, changing its direction. All lenses, whether for telescopes or eyeglasses, are designed to take advantage of this fact. By changing the direction of travel of a light image, a lens deceives the eye and brain into giving the impression that the image is coming from a fictitious direction. Light traveling from one substance to another will be bent varying amounts, depending on the densities of the substances involved. The greater the difference in density the greater the change of direction. Many mineral species can bend or refract light in two directions at the same time. The light beam is actually split into two parts by each part being bent a different amount. Refraction and double refraction are caused by the way a mineral's atoms affect light. Each mineral, then, has its own kind and amount of refraction or double refraction which can be measured and used for identification purposes.
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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