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.

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.

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.