Part 4

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.

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.

_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

TOURMALINE

Because of its great color range, which includes pink, green, blue, yellow, brown, and black in many different shades and combinations of shades, tourmaline is one of the most popular of the colored gemstones. Tourmaline with a color near emerald green is particularly popular.

Chemically, tourmaline is a very complex borosilicate, and its color is determined by the various elements present in it. Tourmaline crystals having sodium, lithium, or potassium are either colorless, red, or green; those having iron are blue, blue-green, or black; and those having magnesium are colorless, yellow-brown, or blackish brown.

Some crystals of tourmaline are of two colors, and stones of mixed colors, such as pink and green, can be cut from these. The color mixing may show as zoning with the core color of the crystal overlaid by another color and perhaps even additional layers of other colors. Zoned crystals with a core of deep pink covered by a layer of green have been called “watermelon tourmaline.” Because its refractive index of about 1.6 is too low to give it marked brilliance, and its color dispersion is too low to give it fire, the tourmaline relies almost solely on the beauty of its color for its rank in popularity.

Although tourmaline has a low refractive index and low dispersion, it exhibits remarkable dichroism. In other words, it can present different tints to the viewer depending on the direction that the light is traveling through the crystal. When viewed down the long, or vertical, axis of the crystal, the color of tourmaline is much stronger than when viewed from the side. This means that if the crystal is dark the cutter will have to cut the stone with the flat part, or table, parallel to the long axis of the crystal. The color of the gemstone then will be lightened when viewed from its table, since this is the direction of lighter color. Similarly, the table of a lighter colored crystal can be cut perpendicular to the long axis in order to produce a deeper colored gem.

Some tourmaline crystals contain threadlike tubes or inclusions of microscopic size running parallel to its length. When cut as cabochons, such crystals give a good “cat’s-eye” effect.

Tourmaline has no distinct cleavage and has a hardness somewhat above 7, and these characteristics make the stone sufficiently resistant to normal shock and wear so that it is highly satisfactory for use in jewelry.

Noted deposits of tourmaline are located in the Ural Mountains of Russia, Ceylon, Burma, South-West Africa, Madagascar, Brazil, Maine, and California. Crystals from each of these localities seem to have their own color specialties. The deposits in San Diego County, Calif., are unique in that all colors except brown are found there. In the early 1900’s pink and red tourmaline was shipped from there to China for carving, but this thriving trade stopped with the end of Chinese imperial reign. The tourmaline deposits at Paris, Auburn, and Hebron, Maine, have furnished a number of excellent gems, especially of blue and green colors.

VARIETIES Achroite: Colorless Indicolite: Blue Dravite: Brown Schorl: Black Rubellite: Pink

ZIRCON

Zircon, because of its high refractive index and high dispersion, approaches diamond in degree of brilliance and fire. On only casual examination it is quite possible to mistake a well-cut, colorless zircon for a diamond. However, a careful examination of the back facets of such a stone, when viewed through the table, would show strong double refraction, a characteristic of zircon but not of diamond. Zircon’s double refraction makes the back facet edges appear doubled. Since diamond is “singly refracting,” it cannot produce this double appearance of the back facets.

Zircon is brittle and has a hardness of just over 7, while diamond’s hardness, as we have seen, is rated at 10. After being worn in jewelry for a long period of time, zircon will show signs of chipping on the facet edges. Under the same conditions, diamond would remain unchanged. Because of this tendency for facet edges to chip, it is the practice in the gem trade to pack cut zircons separately. If a number of zircons were placed in the same paper packet there would be a risk of “paper wear.”

In the gem trade, the most important zircons are those that are colorless, golden brown, or sky blue. Such stones originally were reddish brown zircon pebbles from Indochina, but they have been converted by being subjected to temperatures approaching 1800° F. for periods of up to two hours. When the original zircons are heated in a closed container, the stones become blue or colorless; when a flow of air is allowed through the container, the stones become golden yellow, red, or colorless. In most of these converted stones the color remains quite stable, but in some it may revert to an unattractive greenish or brownish blue after a period of time.

In addition to being reddish brown, natural zircon may vary from almost colorless to yellow, red, orange, and brown or from yellow-green to dark green and, occasionally, blue.

The most important producing areas of gem zircon are in a region of Indochina that comprises parts of Thailand, Viet Nam, and Laos. Additional gem zircon, like so many of the other gem species, is recovered from near Moguk in Upper Burma and from the gem gravels of Ceylon.

There is no synthetic zircon on the market, but a bright blue synthetic spinel is sometimes used to simulate zircon successfully.

PERIDOT

The relative rarity of peridot and the ease with which it can be simulated in glass, whose luster it approximates, probably account for the low popular demand for this gemstone. Although peridot has little brilliance and no fire, its unusual color and glassy luster produce a unique effect that serves to make it attractive.

The color of peridot is an unusual bottle green that shades, in some stones, toward yellow-green and, more rarely, toward brown. In 1952 it was discovered that almost all of the brown gems believed to have been peridot in various gem collections were actually of an entirely unrelated species, which since has been named sinhalite. Brown peridot still remains rare and is somewhat of a collector’s item.

The green of peridot, which is quite different from the green of other gemstones, is due to some iron included in its composition. It is suspected that a trace of nickel contributes to the liveliness of the color.

Peridot has a hardness of only 6½ and a rather strong tendency to cleave, and these characteristics reduce its value for use in jewelry exposed to rough wear. It is better used in pins, earrings, and pendants than in rings.

Peridot is a gem name for the common mineral olivine, a magnesium silicate. Olivine is fund in numerous places, and small gemmy pieces are found in many localities. Many of the largest and best gems of peridot have come from mines on the Egyptian island of Zebirget (Island of St. John) in the Red Sea, but most gem peridot now comes from Burma. Great numbers of small stones have been cut from olivine found in Arizona gravels.

Centuries ago, peridot was known by the name topaz, since the stones came from Topazos, the island now known as Zebirget. The name topaz, as we have seen, is used today for an entirely different mineral species.

SPODUMENE

Spodumene, a lithium aluminum silicate, is one of the very few gemstones containing lithium. It has had more importance as a gemstone in the United States than elsewhere, a situation due to early discoveries of unique occurrences of a lavender-pink variety at Branchville, Conn., in 1879 and in San Diego County, Calif, about 20 years later. At the time of the discovery of the California material, the variety was named _kunzite_ in honor of G. F. Kunz, a noted American gemologist of the times.

The finding of a bright green variety, _hiddenite_, in North Carolina about 1880 greatly stimulated the interest of American gem collectors. Some of the bright green spodumene coming from Brazil in recent years compares very favorably in color with North Carolina hiddenite. Other than in a scattered few of these unusual occurrences of kunzite and hiddenite, spodumene usually is found in yellow and yellow-green shades, with Brazil and Madagascar being the chief sources.

Spodumene has a hardness of about 7, but with a refractive index of about 1.66 and a low dispersion there seems to be relatively little to recommend it as a gemstone. The fact that it exhibits a very strong tendency to cleave in two different directions would seem to rule it out completely as being too difficult to cut. Nevertheless, the production and purchase of cut stones of spodumene persist because of the beauty of the gem.

The kunzite and hiddenite varieties of spodumene show strong _pleochroism_, or the ability to show three different colors when viewed in the direction of different axes. Some of the large Brazilian kunzite crystals mined in the early 1960’s have an intense rose-violet color when viewed along the long axis of the crystal but have pale blue-violet and pale tan colors when viewed from the other two directions. When heat treated, or exposed to strong light, this Brazilian kunzite loses its tan and bluish colors but retains the intense rose-violet. Because of spodumene’s pleochroism, the direction of cutting in the stones becomes extremely important, as it must be done in a manner that will take advantage of the violet color in kunzite and the green color in hiddenite.

VARIETIES Kunzite: Lavender violet to rose violet Hiddenite: Deep green

GARNET

The name garnet is applied to a group of six closely related silicate minerals that are alike in crystal structure but that differ mainly in the substitution of certain metallic elements in their composition. These minerals are:

_Pyrope_, magnesium aluminum garnet _Almandine_, iron aluminum garnet _Spessartine_, manganese aluminum garnet _Uvarovite_, calcium chromium garnet _Grossular_, calcium aluminum garnet _Andradite_, calcium iron garnet

Most natural garnets have compositions intermediate between members of the basic group of six. For example, there are garnets having compositions anywhere between pyrope and almandine, depending on the amount of difference in the magnesium or iron content. These same garnets may even have varying amounts of manganese, and thus be partially spessartine.

The six garnets in the basic group are found in considerable quantity in many areas, but seldom are they of sufficiently high quality to be considered gemstone material. Even when stones of gem quality are found, their colors—particularly the reds—tend to be so intense that they seem to be opaque.

Garnet has a hardness (about 7) suitable for gemstone material and a fairly high refractive index (1.74 and above).

Ruby red pyrope is the most popular variety of garnet. It is found in Bohemia, in Czechoslovakia, where it occurs as small, poorly shaped crystals. Red pyrope also is found in Africa, where it is called Cape ruby, and in Arizona, where it is sold as Arizona ruby. Another kind of pyrope called _rhodolite_ is noted for its soft, rosy purple color. Actually, rhodolite is one of the intermixed garnets with a composition somewhere between pyrope and almandine. Most of the fine rhodolite gems have come from North Carolina.

Almandine is popular in its deep red, transparent form, but since the red is so dark and intense that it appears black, the stones usually are cut as cabochons with the back hollowed out. This makes them thinner, and thus lightens their color. Garnets cut in this manner are all known as carbuncles. Brazil, India, Ceylon, Australia, and parts of the United States are important sources of almandine.

Although spessartine has a rich orange color, it is not often used as a gemstone because of the relative rarity of gem-quality cutting material. This mineral gets its name from the town of Spessart, Germany, where it was first found. Excellent spessartine with colors ranging from orange to brown has been found at Amelia Court House, Va., and quality gems have been cut from such material. Ceylon, Burma, Madagascar, and Brazil also have furnished some gem spessartine.

The chromium garnet, uvarovite, generally is too poor in quality for cutting. Uvarovite crystals, which are emerald green in color, occur in only small sizes. They are found mostly in Russia, Finland, and California.

Grossular varies in color. It occurs chiefly in some shade of red, green, yellow, or brown, depending on the impurities present. When pure, grossular is colorless. A kind of grossular called _hessonite_ has an attractive cinnamon color, and it is found mainly in Ceylon. Because of its color it can easily be confused with spessartine, which it closely resembles.

Andradite, a very common garnet, usually is found in shades of red, black, brown, yellow, or green. Some types of gem andradite have special names for different colors: _topazolite_, yellow; _demantoid_, green; and _melanite_, sparkling black. The very valuable demantoid is found in Russia and Italy.

VARIETIES: Grossular: Colorless, green, amber, brownish yellow, rose Hessonite: Cinnamon colored Pyrope: Deep red Rhodolite: Rose red and purple Almandine: Deep red Spessartine: Brownish red to orange Andradite: Yellow, greenish yellow, emerald green, brownish red, brownish yellow, brown, black Topazolite: Yellow to greenish Demantoid: Grass green to emerald green Melanite: Black Uvarovite: Green

JADE

The name jade is applied to two unrelated minerals—_nephrite_ and _jadeite_—that have somewhat similar characteristics.

Jadeite, the rarer of the two, is a sodium aluminum silicate that belongs to a group of rock-forming minerals known as pyroxenes. Its color varies from white to emerald green and many other colors. Jadeite is highly prized, and when it occurs as emerald green it is considered one of the most valuable gemstones. This kind of jade is found in many places, but the most important occurrence is in Upper Burma. Nephrite, a more common species, is a calcium magnesium iron silicate belonging to a group of rock-forming minerals known as amphiboles. The color varies from white to a dark spinach green and black. Among the places where nephrite occurs are New Zealand, Turkestan, Siberia, Alaska, China, Silesia, and certain parts of the western United States, notably in Wyoming and California.

Jade is not particularly hard (6½), but it is very tough, and this characteristic makes it an excellent material for carving. Even when subjected to punishing usage, jade resists chipping and wear. It was used for making tools and weapons by primitive peoples who lived in what is now Mexico, Switzerland, France, Greece, Egypt, Asia Minor, and in other places. The jade implements fashioned by these peoples have survived well the ravages of time.