Idiochromatic Gems

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.

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.

Light Characteristics

Light is easier to understand in terms of what it does than of what it is. The things it does are about as varied as the things it touches. Each substance has its own unique composition and structure, and handles light in a difway. When the word light is used it y means visible light. Visible light is ly only a very small part of the total y which is of a type known as electromag-radiation. Solid substances have an effect of this electromagnetic radiation. Their julation of visible light is of greater in when dealing with gemstones. Some the reaction between gemstones and ; light produces phenomenal and often full or exotic results. Some of these i will be discussed here are color, schiller, >m and chatoyancy, fire, and fluorescence.

color: Color, or perhaps even its absence, sn the most striking characteristic of a one. Basically, the color of a solid object ids on how it absorbs, transmits, or re-the various wavelengths or colors of light Lich it is exposed. "White" light is com-of a whole series of wavelengths or colors. 1 it strikes a solid object some of the colors be absorbed by the atomic structure as I energy. If this absorbed energy is con-1 to heat and then dissipated it is, in , lost to the observer. However, there is possibility that it will be converted to a ent visible wavelength and passed on . Assuming that some of it is absorbed, or all of the remaining wavelengths are :ed or reflected at the surface and come to the observer as something other than :, because some wavelengths have been acted from the original. Perhaps some or ill travel through the gemstone or diamond wedding bands instead of eye. If all wavelengths are absorbed, no light is reflected or transmitted and the solid looks black. If all are partially absorbed in an equal amount, the stone reflects or transmits gray.

Silicates

Aquamarine, emerald, tourmaline, topaz, zircon, peridot, spodumene, and garnet are gem minerals known by the general name "silicates" and contain both silicon and oxygen as their major constituents. Ruby, sapphire, chryso-beryl, and spinel are "oxides," containing oxygen as a major constituent. As already mentioned, each kind of atom is limited in the kind and number of other atoms it can join. The chemist that are also making diamond engagement rings, through the years, has learned to predict the possible combinations and has developed very accurate methods of checking them in the laboratory. He can make almost innumerable combinations of elements in the laboratory, predicting in each case how they will combine with each other. He can also determine the kinds and relative quantities of atoms present in a mineral sample and use a standard method of noting them. His typical analysis of a mineral might show that there are equal numbers of zirconium (Zr) and silicon (Si) atoms present and four times as many oxygen (O) atoms. His notation, then, would read ZrSi04. This is the chemical notation or formula for the gem mineral zircon. Thus, the formula for aquamarine is Be3Al2Si6Oi8—a beryllium aluminum silicate; for chrysoberyl it is BeAl204—beryllium aluminum oxide.

All this seems simple enough until it develops that the chemical formulas for ruby and sapphire are identical—A1203. If absolutely pure, this aluminum oxide, A1203, is colorless. Ruby, however, is red, and sapphire by definition is any color except red. As the formula indicates, they are actually the same mineral, but ruby is aluminum oxide containing very small traces of the element chromium which cause it to have the red color. Sapphire seems to get its colors from tiny traces of iron or titanium, or both together. Certainly, this is a case where chemical impurities gathered by a mineral during its formation produce highly desirable results.

Diffraction Of Gems

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.

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.

Dispersion

Refraction can also cause interesting color effects. The amount of refraction that take's place depends in good part on what the wavelength of the light is. Blue, for example, is bent more than red. This means that a ray of white light, composed of all colors, by extreme refraction can be separated into its parts and sorted out into a rainbow of colors. The phenomenon is known as dispersion. Some mineral structures cause greater dispersion than others. The phenomenon is seen almost at its best in diamond which, because of its high dispersive ability, kicks back a dazzling shower of separated color splashes or fire whenever struck by a beam of white light. Rutile and sphene lack most of the other fine gem characteristics of diamond, but they do well in matching its dispersion. Quartz and glass make poor substitutes for diamond because they have so little dispersion. Zircon is a commonly used substitute because it has the fire flashes of high dispersion and hardness and clarity, as well.