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.