Thursday, August 14, 2008

Refractive Index

It has already been explained how minerals refract or bend a beam of light. As the light hits the flat surface at an angle, it bends upon entering the gem. If the light beam's direction is slowly changed so that it comes to the surface of the gem at an increasingly lower angle, eventually a point is reached where it ceases to bend sufficiently to enter the gem. It just grazes the surface. Any further lowering of the beam causes it to be totally reflected away from the gem. The grazing angle is called the critical angle, and it will differ with each gem substance according to its refracting ability.

An instrument, the gem refractometer, has been devised to measure this critical angle quickly and easily. The instrument usually contains a built-in scale from which the refracting ability, or "refractive index," of the gem can be read directly. To read the index on a typical gem refractometer, of which there are several models on the market, one of the polished faces of the gem is placed against a polished piece of very highly refracting glass mounted in the instrument. Good contact is assured by placing a drop of highly refracting liquid between them. A light beam is brought through the glass to the gem. Any of the light coming to the gem from an angle at which it will be refracted is bent into the gem, away from the instrument, and is lost. Light coming in at an angle beyond the critical is reflected back into the instrument, hits the viewing eyepiece, and its trace shows as a bright section on the scale. The numerical marking on the scale dividing the light portion—representing reflected light—and the dark portion—representing the lost light refracted into the gem—is the critical angle. For convenience, this is numbered on the scale as the refractive index. The measurement is sufficiently precise so that, by consulting a table listing the refractive indices of gemstones, one can usually make a quick identification of the gem in question.

There are other methods for determining refractive index that are somewhat less convenient. One of the best of these uses a series of fluids of known refractive indices. When a gem is placed in a liquid having the same refractive index it effectively disappears. This happens because the gem does not cause any further bending of light than that already done by the liquid. We have no visual evidence of the gem's presence because it behaves toward light exactly as the liquid does. And also round diamond earrings An appropriate series of test liquids such as clove oil (index 1.54) to test for quartz (index 1.54), cassia oil (index 1.60) to test for topaz (index 1.61) and methylene iodide (index 1.74) to test for spinel (index 1.72-1.73) is easily assembled. Although some of the liquids are expensive and difficult to obtain and may be unstable enough to need frequent replacement, the method has some benefits for example diamond solitare earrings. It can be used with very small gem-stones fragments, and the flat, polished surfaces required for the refractometer are not necessary. Also, even when the gem material disappears in a liquid of appropriate index, the cracks, flaws, and foreign inclusions in it do not vanish. This affords an excellent method of checking a gemstone internally to see how it can best be cut to make the most perfect gem. It also may make it easier to see and study the nature of the inclusions.

Tuesday, August 12, 2008

Electrical Properties

Gems may have a number of other interesting characteristics unrelated to their appearance or durability. Among these, electrical behavior is sometimes remarkable. Museum curators and jewelers have long known that their tourmaline specimens and gems will accumulate thick coats of fine dust in a short period of time even when they are displayed in tightly sealed showcases. As the case lights are turned on and off each day the tourmaline is alternately heated and cooled. When heated, tourmaline develops a substantial electric charge which quickly attracts tiny dust particles in the air. Before long, a gem will be coated. This characteristic is known as pyroelectricity—electricity produced by heating. Diamond, topaz, tourmaline, and amber, when polished briskly with a cloth, will even develop enough of an electric charge to attract and hold small bits of paper.
An electrical effect discovered by Pierre and Jacques Curie in France in 1880 is perhaps even more remarkable. These two physicists, studying the ability of crystals to conduct electricity, found that when certain crystals were squeezed they developed a measurable electrical charge. The phenomenon was later called "piezoelectricity," based on a Greek word piezin, meaning "to press." As early as 1881 another Frenchman, G. Lippman, predicted that a reverse phenomenon would take place. He suggested that an electric charge placed on any piezoelectric crystal would cause it to change shape. This was successfully tested by the Curie brothers. Today this important characteristic is applied, using quartz and other piezoelectric substances, to control the frequencies of all radio broadcasting and other electronic devices. The alternating current in these devices and the piezoelectric crystal plates inserted in them must operate in unison. Thus, the fixed dimensions and structure of the plate keep the current alternating only at the rate at which the crystal plate can change its shape. Such a device is known as a frequency control oscillator. It is responsible for the fact that every time you tune in your favorite radio or television station it is at the same number on the dial, just where it was last time.

Friday, August 1, 2008

Weight and Specific Gravity

: Contrary to popular understanding, the fact that a gem weighs, say, ten carats has little to do with its size, except when compared with a gem, 1 carat diamonds or 2 carat diamonds of its own species and of similar cut. The carat is an expression of weight, not size. One carat is equal to one-fifth of a gram; in more familiar units, there are about 140 carats in an ounce. This standard unit of measurement of gem-stone weights was not legalized in the United States until 1913. Before 1900 the several weight units used throughout the world differed somewhat, but the carat in one form or another had been used since ancient times. Speculation relates the weight of the carat to the weight of a seed of the tropical carob tree. These seeds are unusually constant in weight and are just slightly lighter than our legal metric carat. The ancient Greek weight—the ceratium—and the Roman siliqua are both about the same as the carob seed. Most likely, then, the name and the weight did originate with the carob tree.

Even knowing that the carat is an expression of weight only, gem buyers are often surprised to see that a one-carat sapphire is considerably smaller than a one-carat diamond. Sapphire is denser than diamond and a smaller stone can weigh the same number of carats as a larger diamond. Since size and weight are both very important measures for gemstones, the unit of .measurement that combines both— "density"—is fundamental.

It is complicated to think of a gem's density, because size and weight must be considered simultaneously. We are aware of a difference in weight when we compare iron and wood. Yet it would not always be correct to say that iron weighs more than wood because a large piece of wood can weigh more than a small piece of iron. Only by comparing equal volumes of these materials can the extent of the weight difference be made clear and unmistakable. Making innumerable comparisons of the weights and volumes of different solids with each other seems inconvenient. To avoid this, it is customary to compare all of them with some selected substance, readily available, with a known density. Water is the standard most often used. Ruby, for example, proves to be four times heavier than an equal volume of water, so its specific gravity—the term of comparison with water—is 4. Diamond weighs SlA times as much as an equal volume of water and thus has a specific gravity of 3Vi.