Gems in the Smithsonian Institution

by Paul E. Desautels

SPINEL

Two of the more famous stones in the British crown jewels are the Black Prince’s Ruby and the Timur Ruby, but neither of these stones is really ruby. Like the great red gem in the crown that belonged to the Russian Empress Catherine II, these two British stones are spinel. Although spinel occurs in many colors, such as yellow, green, violet, brown, and black, it is the red spinel that usually is seen in the gem trade. There are several varieties of red spinel, such as ruby spinel, balas ruby, rubicelle, and almandine spinel—all of which refer to the color resemblance to ruby.

The hues and tints of spinel show subtle variations that are matched only by those of tourmaline. Unlike tourmaline, however, spinel may be bright ruby red. The cut stones curving around two pieces of rough from Burma weigh (left to right) 30 carats (Ceylon), 34 carats (Burma), 36 carats (Burma), 30 carats (Ceylon), and 22 carats (Ceylon). (Three-fourths actual size.)

Spinel is an oxide of magnesium and aluminum, and it is not related to ruby. However, because its hardness (8) is only slightly less than that of ruby and its brilliance is about equal to that of ruby, spinel makes an excellent substitute for that gem. Also, because it is more plentiful, spinel costs much less. It is interesting that red spinel, like ruby, gets its color from the presence of traces of chromium.

Synthetic blue spinel is widely used as a substitute for aquamarine, and synthetic spinels of other colors are used as substitutes for many gems. However, the synthetic stones are not ordinarily made in the subtle shades so characteristic of natural spinel. Completely colorless spinel apparently exists only as a synthetic material. Actually, because of its hardness, durability, and many attractive colors, spinel makes a fine gemstone in its own right.

Like ruby and several other gemstones, spinel is found chiefly in the gem gravels of Ceylon, Burma, and Thailand. Appreciable amounts of spinel occur in the Ceylon gem gravels as worn, rounded pebbles of many colors. In the Burmese gravel deposits the spinel is often found as well-formed octahedral crystals. Near Moguk, in Burma, spinel has been found in its original position in the limestone rocks as well as in the eroded stream deposits.

VARIETIES Almandine spinel: Purplish red Rubicelle: Orange-red Balas ruby: Rose red Ruby spinel: Deep red Chlorospinel: Translucent grass green Ceylonite or pleonaste: Opaque dark green, brown, or black Picotite or chrome spinel: Translucent dark yellow-brown or green-brown

QUARTZ
(INCLUDES ROCK CRYSTAL, AMETHYST, AND CITRINE)

Few gemstones can compete with quartz for variety of color. Having a hardness of 7 and occurring in many beautiful varieties, only the relative abundance of quartz prevents the species from attaining top rank among gemstones.

The two kinds of quartz, crystalline and cryptocrystalline (fine-grained) quartz, occur in all kinds of mineral deposits throughout the world. Much of this material is suitable for cutting gems.

Colorless crystalline quartz, or rock crystal, makes attractive faceted gems, and it is used as a suitable substitute for diamond and zircon even though it lacks the fire and brilliance of those gemstones. Some very large, flawless crystals of colorless crystalline quartz have been found. The great Warner Crystal Ball, with a diameter of 12⅞ inches and weighing 106¾ pounds, was cut from such a crystal. In addition to the name rock crystal, colorless crystalline quartz appears in the jewelry trade under such names as rhinestone (not to be confused with the glass substitute), Herkimer diamond (from Herkimer County, N. Y.), and Cape May diamond (from Cape May, N. J.).

The most popular variety of quartz is amethyst, a transparent form whose color ranges from pale violet to deep purple. In many cut stones of amethyst the color intensity changes sharply from section to section. This is due to irregular color zoning common to amethyst crystals. The actual cause of the purple color in amethyst is not very well understood. There are fewer cut stones of amethyst in very large sizes because of the rarity of large, flawless, well-colored crystals.

This 4500-carat pale smoky quartz egg from California rests on a gold stand set with Montana sapphires. The unique gem was cut and its stand was designed and made by Capt. John Sinkankas as a difficult exercise in the lapidary art. The quartz egg is 4 inches long and almost 3 inches in diameter.

The name citrine (from the French word for lemon) attempts to describe the yellow color of another variety of quartz. Actually, the normal coloring of citrine varies from yellow to red-orange and red-brown, but the yellow sometimes rivals the yellow of topaz. In addition to the normal color range, the colors of citrine may grade through a grayish yellow variety known as cairngorm and a grayish variety called smoky quartz to a black variety called morion. Other varieties that add color dimensions to the group of quartz gemstones are rose quartz and milky quartz. Like amethyst, the reason for the color in rose quartz has not been definitely established. Milky quartz owes its color to myriads of tiny cavities containing water or liquid carbon dioxide.

A 783-carat step-cut citrine of deep, rich color dwarfs a 278-carat brilliant-cut citrine (at left), a 90-carat smoky quartz, and a 91-carat briolette of citrine. The smoky quartz, from Switzerland, is so dark that it appears to be opaque. The other three stones came from Brazil. The briolette and brilliant-cut citrines were cut and donated to the Smithsonian Institution by Albert R. Cutter. (Slightly less than half actual size.)

The range of color in quartz is somewhat surprising, considering that the mineral is a simple silicon dioxide. Some of the colors, as with corundum and some other gemstones, are due to traces of impurities. In quartz, these consist mainly of oxides of iron, manganese, and titanium. However, all the reasons for quartz coloration in its many varieties are not known.

Pastel rose quartz has a delicate beauty in any cut. The 375-carat step cut (top), the 84-carat step cut, and the 46-carat marquise came from Brazil. (Two-thirds actual size.)

Amethyst, a purplish quartz, is the birthstone for February. Here it is represented by a 1362-carat stone from Brazil (top), a 54-carat stone from Pennsylvania (left), and a 21-carat stone from North Carolina. (Almost actual size.)

In addition to possessing wide variation of color, quartz, like sapphire and certain other gemstones, can exhibit asterism or chatoyancy. The well-known tiger’s-eye from West Griqualand, South Africa, owes its eye effect to the fact that its material is a replacement of fibrous asbestos by cryptocrystalline quartz. The color of tiger’s-eye arises from the partial alteration of the asbestos to yellow-brown iron oxides before it is replaced by quartz. Inclusions of rutile, tourmaline, or actinolite needles may produce attractive patterns in quartz, but they do not always cause chatoyancy. The material containing such inclusions is called sagenitic quartz, or it may be descriptively named, such as rutilated quartz, tourmalinated quartz, and so forth. Sagenitic quartz is usually cut as cabochons rather than as faceted stones since the inclusions are of greater interest than the quartz itself.

If the foreign inclusions consist of tiny flakes of hematite or mica, the quartz assumes a spangled appearance and is called aventurine.

Crystals of quartz varieties that are opaque or that contain visible inclusions normally are cut as cabochons to take advantage of the body color or to make the inclusions more visible. Crystals of the transparent varieties are fashioned in any of several cutting styles, depending on whether it is desired to take maximum advantage of color or of brilliance. Because of its availability in fairly large, flawless pieces in various colors, quartz has been used extensively in carving. The Chinese have excelled in carving large, ornate objects of rock crystal.

Although quartz occurs in many varieties and its crystals are cut in many styles, it is easily identified by its refractive index of 1.55, specific gravity of 2.65, and hardness of 7.

CRYSTALLINE VARIETIES Amethyst: Purple to violet Cairngorm: Smoky yellow Citrine: Yellow to red-orange and red-brown Milky quartz: White Morion: Black Rock crystal: Colorless Rose quartz: Rose to pink Smoky quartz: Gray to black

CRYPTOCRYSTALLINE VARIETIES (CHALCEDONY) Agate: Pronounced color banding Aventurine: Inclusions of sparkling flakes Bloodstone: Dark green dotted with red Carnelian: Red to yellow-red Cat’s-eye: Chatoyant Chrysoprase: Green Jasper: Opaque brown to red-brown, green, yellow, etc. Onyx: Color banding in straight layers of contrasting color Sard: Light to dark brown Sardonyx: Sard or carnelian bands alternating with white bands Tiger’s-eye: Bright brownish yellow, sometimes blue: chatoyant

CHRYSOBERYL
(INCLUDES ALEXANDRITE AND CAT’S-EYE)

With color ranging from shades of yellow and brown through blue-green to olive, and with a hardness of 8½, chrysoberyl has most of the characteristics necessary for a fine gem. Rare stones of high-quality chrysoberyl demand fairly high prices, and they are sought eagerly by the connoisseur of gemstones.

Chrysoberyl is beryllium aluminate, and thus is closely related to the gemstone spinel, which is magnesium aluminate. When pure, chrysoberyl is colorless and relatively uninteresting as a gemstone because of its lack of color dispersion and its moderate refractive index of 1.75. However, few pure samples are known, as chrysoberyl normally contains some iron or chromium in place of aluminum and some iron in place of beryllium. As a result of such impurities, the color of chrysoberyl my be yellowish, greenish, or brownish.

Chrysoberyl and beryl are the only important gemstones containing the element beryllium. The minerals beryllonite, euclase, hambergite, and phenakite also contain this element, but they are rare and seldom are seen as cut gems.

One of the finest chrysoberyl cat’s-eyes in existence is the 58-carat Maharani from Ceylon. (Actual size.)

The alexandrite variety of chrysoberyl has two colors in delicate balance, and it changes from a columbine red to an emerald green when viewed under different light. When viewed in daylight, which is richer in green, the color balance shifts toward green, and that hue is seen by the observer. Under artificial light, normally richer in red, the color balance shifts toward red, and the stone seems to have changed to that color. This extremely rare stone, named after Czar Alexander II of Russia, is found only occasionally, in Russia and Ceylon. The Russian stones, found with emerald in mica schist, tend to be smaller than the Ceylon stones and have a color change going from emerald green to violet-red. The Ceylon stones, found as pebbles in gem gravels, have a color change going from a less-emerald green to a browner red. The 66-carat, record-size alexandrite in the National Collection shows the color change typical of Ceylon stones. A synthetic stone is commonly marketed as synthetic alexandrite, but this substitute not only is man-made but is actually synthetic corundum instead of synthetic chrysoberyl.

In addition to its fine cat’s-eyes and its color-changing alexandrite varieties, chrysoberyl occurs in handsome stones that vary in depth of color. Shown here with an uncut twinned crystal of gem quality from Brazil are a 46-carat stone from Brazil (left) and a 121-carat stone from Ceylon. The uncut crystal is a gift of Bernard T. Rocca, Sr. (Two-thirds actual size.)

Cat’s-eye chrysoberyl contains myriads of tiny fiberlike channels arranged in parallel position. When the stone is cut as a cabochon, a band of light is reflected from the curved top of the stone, producing an effect that resembles the slit pupil of a cat’s eye.

VARIETIES Alexandrite: Green in daylight, changing to red in artificial light Cat’s-eye: Chatoyant