Gems in the Smithsonian Institution

2 the Study of Gems

To the average person it might seem that a jeweler’s showcase of gems presents innumerable kinds of precious stones, when actually only a few species of minerals are there. Perhaps only diamond, ruby, emerald, aquamarine, sapphire, opal, tourmaline, and amethyst would comprise the entire stock. Yet, since the mineral kingdom consists of about 2000 distinct species, it would seem that a few more kinds of gemstones would be available. Certainly, many more minerals than are seen displayed by the jeweler have been used as gems over the centuries. The study of all these species of gem minerals constitutes modern gemology—a specialized branch of the science of mineralogy.

With the few exceptions already noted, all gems are minerals found in the earth’s crust. A mineral is a natural substance having a definite chemical composition and definite physical characteristics by which it can be recognized. However, for a mineral to qualify as a gem it must have at least some of the accepted requirements—brilliance, beauty, durability, rarity, and portability. Of course, if a gemstone happens to be “fashionable” it will have additional importance. Rarely does a single gem possess all of these qualities. A fine-quality diamond, having a high degree of brilliance and fire, together with extreme hardness and great rarity, comes closest to this ideal, and in the world of fashion the diamond is unchallenged among gems. The opal, by contrast, is relatively fragile, and it depends mainly on its rarity and its beautiful play of colors to be considered gem material.

When a gem material, as found in nature, has at least a minimum number of the necessary qualities, it is then the task of the lapidary, or gem cutter, to cut it and polish it in such a way as to take greatest advantage of all its possibilities for beauty and adornment.

Physical Characteristics of Gemstones

When a gemologist or a gem cutter examines an unworked mineral fragment (called rough) he looks for certain distinguishing characteristics that will aid him in identifying the mineral and in determining the procedures he should use in cutting it.

It is difficult to list these characteristics in the order of importance, but hardness would rank high. Hardness of a gem is best defined as its resistance to abrasion or scratching. Most commonly used for comparison is the Mohs scale, which consists of selected common minerals arranged in the order of increasing hardness. On this scale, topaz is rated as 8 in hardness, ruby as 9, and diamond, the hardest known substance, as 10. Any gem with a hardness less than that of quartz, number 7 in the scale, is unlikely to be sufficiently scratch-resistant for use as a gem. A less precise scale, using common objects for comparison, might include the fingernail with a hardness up to 2½, a copper coin up to 3, a knife blade to 5½, a piece of window glass at about 5½, and a steel file between 6 and 7, depending on the type of steel. By this scale, any stone that remains unmarred after being scraped by a piece of window glass will have a hardness greater than 5½. The more important gemstones—which include diamond, ruby, sapphire, and emerald—all have a hardness much greater than 5½.

The size of a gemstone usually is indicated by its weight in carats. The expression “a 10-carat stone” has meaning—if somewhat inexact—even to the nonexpert. Specifically, a carat is one-fifth of a gram, which is a unit of weight in the metric system small enough so that approximately 28 grams make an ounce. A 140-carat gemstone, then, weighs about an ounce.

Another distinguishing characteristic of a gemstone is its specific gravity, which is an expression of the relationship between the stone’s own weight and the weight of an equal volume of water. We are aware of a difference in weight when we compare lead and wood, yet it would not always be correct to say that lead weighs more than wood, for a large piece of wood can weigh more than a small piece of lead. Only by comparing equal volumes of these materials can the extent of the weight difference be clear and unmistakable. Diamond is 3½ times heavier than the same volume of water, so its specific gravity is 3.5. Since each species of gem has its own specific gravity, which can be determined without harming the stone, this standard of comparison is a valuable aid in identifying gems. Several techniques have been devised for determining specific gravity, and most of them make use of some kind of weighing device or balance.

Among the most striking and useful of the distinguishing characteristics of gemstones are those that involve the effects on light.

An important effect of a gem on light is the production of color, upon which many gems depend for their beauty. Some gem materials, such as lapis lazuli, have little to offer except color. Many gemstones vary widely in color, owing to the presence of varying but extremely small amounts of impurities. Thus, the gemstone beryl may occur as blue-green (aquamarine), as pink (morganite), as rich green (emerald), as yellow (golden beryl), or even colorless (goshenite).

Gemstones such as beryl and sapphire that depend on impurities for their color are said to be allochromatic; others, such as peridot and garnet, which are highly colored even when pure, are said to be idiochromatic. The color of a gem is further described according to its hue, tint, and intensity. Hue refers to the kind of color, such as red, yellow, green, etc.; tint refers to the lightness or darkness of the hue; and intensity refers to vividness or dullness. Throughout history, the most popular colored stones have been those with hues of red, green, or blue of dark tint and high intensity.

The effect of a gem on light may be more than the production of color. Several of the so-called phenomenal stones are prized for other effects. Holes, bubbles, and foreign particles, when properly aligned in parallel groupings, can produce interesting light effects. The play of colors of opal and labradorite, the chatoyancy or silky sheen of tiger’s-eye and cat’s-eye, the opalescence or pearly reflections of opal and moonstone, and the asterism or star effect of rubies and sapphires are caused by the reaction of light to minute inclusions or imperfections in the gemstone.

When light passes into or through a gemstone with little or no interruption, the stone is said to be transparent, as opposed to a stone through which light passes with greater difficulty, and which is said to be either translucent or opaque, depending on the degree of light interruption.

The action of a gemstone upon the light which strikes its surface and is either reflected or passed through it sometimes results in highly desirable effects that enhance its beauty and aid in its identification. Light passing into a stone is bent from its path, and the amount of bending (refraction) depends upon the species of the gemstone. When the degree of bending can be measured, the gem species can be identified, since very few species of gemstones bend light to exactly the same degree. An instrument called a gem refractometer is used to determine the degree to which cut stones refract, or bend, light. The measurement obtained is the refractive index of the gemstone.

Many gemstones can split a beam of light and bend one part more than the other, thus producing double refraction, or two different measurements of refractive index.

Gems have the ability to separate “white light” (the mixture of all colors) into its various colors, producing flashes of red, yellow, green, and other colors. Separation occurs because the various colors, or wavelengths composing white light passing through the gem, are each bent or refracted a different amount. Red is bent least, followed in order by orange, yellow, green, blue, and violet, which is bent most. This characteristic of being able to produce flashes of color, as seen prominently in diamond, is known as dispersion or fire. Quartz and glass have low dispersion, and hence they make poor diamond substitutes. Some of the newer synthetic gemstones, such as titania, have extremely high dispersion, with resulting fire. Zircon, a natural gemstone of suitable hardness, exhibits high dispersion and is a commonly used substitute for diamond.

Significantly, ruby and sapphire are chemically identical, both being of the mineral species corundum. As already explained, the difference in color is due entirely to very slight traces of chemical impurities. Frequently, the impurities are present in irregular patches that give spotty color effects.