Part 1

_Gems_ _in the_ SMITHSONIAN INSTITUTION

by PAUL E. DESAUTELS

_Associate Curator_ Division of Mineralogy

WASHINGTON, D. C. 1965

SMITHSONIAN INSTITUTION PUBLICATION No. 4608

LIBRARY OF CONGRESS Card No. 65-60068

CONTENTS

The National Gem Collection 1 The Study of Gems 3 The Shaping of Gemstones 10 Gem Substitutes 20 Gem Lore 24 The Principal Gem Species 27 Some Notable Gems in the Collection 70

1 THE NATIONAL GEM COLLECTION

Man has been using certain mineral species for personal adornment since prehistoric times. However, of the almost 2000 different mineral species, relatively few, perhaps only 100, have been used traditionally as gems. To be used as a gem, a mineral species must have durability as well as beauty. Lack of durability eliminates most minerals as gems, although some relatively fragile gem materials such as opal are prized because of their exceptional beauty. Actually, some gem materials are not minerals at all. Pearl, amber, jet, and coral are formed by living organisms.

In the National Gem Collection, the Smithsonian Institution has assembled a large representation of all known gem materials. The display portion of the collection consists of more than 1000 items selected to illustrate the various kinds of gems and to show how their beauty is enhanced by cutting and polishing. All of these gems are gifts of public-spirited donors who, by giving the gems directly or by establishing endowments for their purchase, have contributed to the enjoyment of the many thousands of persons who visit the Smithsonian Institution each week.

The National Gem Collection had its beginning in 1884 when Prof. F. W. Clarke, then honorary curator of the Division of Mineralogy, prepared an exhibit of American precious stones as a part of the Smithsonian Institution’s display at the New Orleans Exposition. The same collection was displayed at the Cincinnati Exposition the following year. Between 1886 and 1890 the growth of the collection was slow, but in 1891 most of the precious stones collected by Dr. Joseph Leidy of Philadelphia were obtained, and these, combined with those already on hand, were exhibited at the World’s Columbian Exposition at Chicago in 1893.

Great stimulus was given the collection in 1894 when Mrs. Frances Lea Chamberlain bequeathed the precious stones assembled by her father, Dr. Isaac Lea. Her husband, Dr. Leander T. Chamberlain, who in 1897 became honorary curator of the collection, contributed a large number of specimens and, upon his death, left an endowment fund. The income from that fund has been used to steadily increase the collection over the years. Extremely rare and costly gems suitable for exhibition are beyond the income derived from the Chamberlain endowment, but this gap has been filled by many important donations, the most notable being the gift of the Hope Diamond by Harry Winston, Inc., New York City. Thus, from modest beginnings in 1884, there has been accumulated the magnificent collection of gems belonging to the people of the United States. The collection is displayed in the Smithsonian Institution’s great Museum of Natural History.

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.

Scale of Hardness

Soft 1. Talc ^ 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Feldspar 7. Quartz 8. Topaz v 9. Corundum Hard 10. Diamond

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.

CHEMICAL CHARACTERISTICS OF GEMSTONES

Since gems are embraced in the mineral kingdom, and minerals are naturally occurring chemical substances, it follows that all the accepted terms of chemical description can be applied to them. When a chemist learns that ruby is an impure aluminum oxide, he understands a great deal about the nature, origin, and behavior of ruby. He can assign to it the chemical formula Al₂O₃, symbolizing its basic composition as two atoms of aluminum united with three of oxygen. Similarly, other popular gemstones can be described chemically as follows:

Diamond Carbon C Sapphire Aluminum oxide Al₂O₃ Quartz Silicon dioxide SiO₂ Emerald Beryllium aluminum silicate Be₃Al₂(SiO₃)₆ Spinel Magnesium aluminate Mg(AlO₂)₂

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.

Some mineral species possess many of the desirable qualities of gemstones yet cannot be used as gems because they are chemically active and therefore are less durable. They undergo alteration and decomposition when exposed to light or to one or another of such substances as air, water, skin acids and oils.

3 THE SHAPING OF GEMSTONES

Gemstone crystals often have naturally brilliant, reflecting faces, but rarely are they perfect and unblemished. Also, their natural shapes do not provide the best expression of their luster, brilliance, dispersion, color, and other inherent properties. In fashioning a gemstone, the skilled artisan tries to develop these hidden assets and to otherwise enhance the gemstone’s general beauty.

From ancient times until the 1600’s little was attempted in the way of shaping gemstones other than to smooth or polish the natural form. Although similarly smoothed, or _tumbled_, gemstones recently have returned to fashion, the finest pieces of gem rough are now converted mainly into _faceted_, or shaped, stones. Standard types of facets—the flat faces that are ground and polished on the rough gem material—have been given individual and group names. A typical example is the _brilliant_ cut, which is most commonly used to best bring out the qualities of a diamond.

The diagram shows a brilliant-cut diamond with angles and facets arranged to give the stone maximum internal reflection as well as to make use of its strong dispersive ability. Certain of the light beams passing into a brilliant-cut diamond produce colorless brilliance by being reflected back out of the stone through the _table_ by which they entered. Other light beams, emerging through inclined facets, are split up by dispersion into the rainbow, or fire, effect so prized in diamonds. A stone that has been cut too wide for its depth, with incorrect facet angles, will look large for its weight but its brilliance and fire will have been drastically reduced.

For other purposes and for other kinds of precious stones a number of basic cuts have been developed. The _brilliant_ and _step_ cuts are by far the commonest of these basic cuts, but modern jewelry design frequently uses such fancy cuts as the baguette, cut-corner triangle, epaulet, half moon, hexagon, keystone, kite, lozenge, marquise, pentagon, square, trapeze, and triangle. Some of these are shown here.

In general, there are three operations in preparing a gemstone from the rough—sawing, grinding, and polishing. Sawing usually is accomplished by using a thin, diamond-impregnated, rapidly rotating disk of soft iron or bronze, with oil or water being used as a coolant. The very hard diamond dust literally scratches its way through the stone. Once the stone is sawed to shape, the facets are ground and polished on a rotating horizontal disk by the use of various abrasives. For rough grinding, silicon carbide—or sometimes diamond powder—is used. Scratches are removed and a high polish is given by the use of tin oxide, pumice, rouge, or other fine-grained abrasives. The thick disks, or laps, are made of cast iron, copper, lead, pewter, wood, cloth, leather, and certain other materials. Since each species of gemstone differs in its characteristics, each must be treated somewhat differently as to sawing and lapping speeds, kind of lap, and choice of abrasives. Because of the greatly increased interest in gem cutting as a hobby and the large number of amateur cutters, a substantial market has developed in the United States for lapidary supplies and equipment. New kinds of machinery, new abrasives, and new kinds of saws and laps are introduced regularly. Fundamentally, however, the process still involves sawing, grinding, and polishing.