Quartz, a multi-faceted mineral, keeps many of our watches accurate and assists in radio, TV and telephone communication. It allows beach-goers to build sandcastles and may even, some believe, have energy-healing capabilities.
But quartz is most important to jewelers because of its beauty and seemingly unending gem variety. At least 15 forms have been given official variety names, without even counting the forms of microcrystalline quartz known as chalcedony.
Quartz seems ubiquitous; its various forms make up approximately 12% of the earth’s outer shell. Though found nearly everywhere, however, much quartz is not of gem quality. That which is happily carriers an affordable price tag. A gemmy amethyst, which is far less rare and valuable than a comparable-quality ruby, may keystone for only $100 per carat. That in itself is part of its beauty; everyone can own a beautiful, natural gem quartz without having to hock their other possessions. Quartz is a wonderful “starter” stone for customers who like gems but hesitate to do any heavy buying. A beautiful ametrine ring or a set of carved rose quartz beads might help such a customer test the waters. Once she’s experienced the joys of owning and wearing colored stones, surely interest in higher-ticket gems will follow.
THE QUARTZ FAMILY
Let’s look at some of quartz’s many varities.
Rock crystal, a transparent, colorless variety of silicon dioxide, is as pure as quartz gets. This was the original “rhinestone” (the word now refers to foil-backed glass imitations of gems), as it was often picked up as waterworn pebbles along the Rhine River. Many misnomers have been applied to rock crystal. Perhaps the best known is “Herkimer Diamond” – derived from the remarkably clear, well-developed crystals found in Herkimer County, New York. Its real name comes from the Greek word Krystallos, meaning “ice,” since the ancients believed rock crystal was ice eternally frozen by an unnatural frost created by the gods.
Rock crystal and other quartz varieties have been used for millennia for some of the finest and most elaborate works of art. The Romans made wine jugs, cups, vases and other articles from large quartz crystals. Lenses of rock crystal were used to concentrate the sun’s rays to generate heat for cauterizing wounds and to light fires. Even crystal balls, perhaps the best-known quartz art objects, date way back into antiquity (the Hindus used “reading balls”). The largest known flawless crystal ball is a 107-lb., 12 3/4-in.-diameter sphere at the Smithsonian Institution’s Gem and Mineral Hall in Washington, D.C.
Sagenite, from the Greek word for “net,” is characterized by eye-visible, needle-like inclusions. The most common included mineral is rutile (thus the name rutilated quartz), which is usually golden brown in color. The rutile needles often are randomly scattered throughout the host – generally rock crystal or smoky quartz – but can take on very specific patterns. If the majority of the long inclusions run in the same direction, a cat’s-eye effect may be seen when the stone is cut en cabochon. (When the needle-like inclusions are very fine, cat’s eye quartz can have an uncanny resemblance to cat’s eye chrysoberyl, the king of chatoyant gems.)
Rutile needles will (rarely) form a six-rayed star. Interestingly, the six-fold symmetry has nothing to do with the trigonal form of quartz. Rather, it is generated from an underlying plate of the mineral ilminite, which crystallizes in the trigonal system itself.
Sagenitic quartz, long esteemed for ornamental purposes, has been known under a number of descriptive names such as fleches d’amour (cupid’s arrows) and Venus hair-stone.
Many other needle-like (acicular) inclusions can be found in this quartz variety, including tourmaline, epidote, actinolite and goethite (pronounced GERtite).
Amethyst undoubtedly is the best known and most valuable quartz variety. Its color can range from purple to bluish purple when viewed in daylight, and from reddish purple to slightly purplish red under incandescent light. (Most amethysts exhibit a moderate color change effect when subjected to various lighting conditions.) The most desirable color is a dark, saturated purple with prominent red flashes – referred to as “Siberian” amethyst, since stones with this color quality have come from Russia. Today, as with most trade names, the term denotes a quality rather than a particular source. Amethyst with a light, pale lilac hue is sometimes called “Rose of France.”
The name “amethyst” finds its root in the Greek word amethustos, meaning “not drunken.” It was believed the stone was a remedy for intoxication. Amethyst evidently was highly popular even among ancient civilizations; intaglio seals and other ornaments are found among the burial crypts and ruins of the ancient Egyptians and Babylonians. Because of amethyst’s supposed power to preserve a wearer from bodily harm in battle, it often was worn as an amulet during the Middle Ages. Both royalty and church officials long favored amethyst. England’s Royal Sceptre is studded with amethysts, and the purple gem often ornaments the altar and its accessories. Those born in February wear amethyst as their birthstone.
A combination of twinning and uneven distribution of the coloring agent (iron) causes color zoning, a common feature of amethyst. Because the zoning can be eye-visible, it is important for the gem cutter to consider orientation. The striated effect, if present, should not be noticed when the stone is viewed in a face-up position. You can see color zoning most easily by putting an amethyst table down against a white background. (In a buying situation, use a business card to perform this quick test.) Careful heating sometimes can permanently “smooth out” color zoning; too much heating, however, can render amethyst (and other quartz varieties) colorless. Heated amethyst also may turn a grayish green color (so-called “greened amethyst”) or brownish yellow to orange. These hues are known as citrine.
Citrine, from the French word for lemon (“citron”), primarily falls within the yellow to orange hue range. The most valuable, commanding up to $70 per carat for a 10- to 25-ct. stone, is medium dark, moderately saturated orange. Another popular color, the so-called Madeira quartz, is reminiscent of reddish orange garnets such as spessartine and malaia.
Many people are surprised to learn that quartz in a citrine color is rarely found in nature. Much of the material we enjoy as citrine starts out as amethyst – which, as noted, can be heated to permanently yield one of the citrine hues. That explains why we so commonly find color zoning in citrine as well as in amethyst.
Citrine for many years has been likened to, compared with and flat out called “topaz”. The most common misnomer associated with it is “topaz-quartz,” no doubt stemming from the ancient practice of calling all yellow stones topaz. This term, like all misnomers, should be abandoned since it does nothing to enhance citrine’s own reputation as a beautiful gem material. In truth, citrine itself has a few advantages over topaz – it is much tougher and much less expensive (a plus to the customer with a limited budget). In an effort to help jewelers promote citrine, the Jewelry Industry Council – prompted by the American Gem Society – included it with topaz as a November birthstone in 1952.
Ametrine is also called trystine, amethyst-citrine or golden amethyst. This parti-colored quartz variety exhibits both purple and brownish yellow hues in the same crystal. Originally reported in 1979 from Brazil, ametrine later was found to occur in Bolivia. Some ametrine occurs naturally; other material is produced by heating some amethyst. (It is not uncommon to find citrine color zonation in amethyst, due to variations in the concentration of iron within the crystal.)
The lapidary who fashions ametrine has his work cut out. As with any color-zoned material, orienting the crystal for the best face-up appearance is crucial. The cutter who works to diminish the visual impact of uneven color distribution in zoned amethyst or citrine, however, purposely cuts ametrine to display its two colors with equal prominence. This is known as designed zoning.
Smoky conjures up tones of gray and brown, doesn’t it? Well, what you say is what you get. The color in smoky quartz is caused by natural radioactivity in the earth. Other names associated with it include Cairngorm (derived from the mountains of the same name in northern Scotland) and morion, which denotes very dark, almost black material.
Not to beat a dead horse, but stay away from the term “smoky topaz.” This misnomer has long (too long!) been incorrectly used to described this quartz variety.
Rose quartz, found in various tones and saturations of pink, is nearly cloudy due to a scattering effect caused by various mineral inclusions. The finest grades of rose are used in jewelry items such as bead necklaces, pins and rings. Lesser transparent material often is carved into objects of art such as vases, perfume vials and paperweights.
Much of the lapidary work in rose and other quartzes is done in Idar-Oberstein, West Germany, which boasts a long tradition of exquisite craftsmanship. A particular favorite among collectors is star rose quartz.
All of the quartz varieties previously mentioned occur in single crystal form, which means they generally are transparent to semi-transparent. A few of the aggregate forms deserve mention, too.
Aventurine, a variety of aggregate quartz (called quartzite), is characterized by a glittery (aventurescent) effect when viewed in reflected light. Concentrations of small included crystals or flat, plate-like mineral inclusions (most commonly green, chrome-rich mica called fuchsite) cause the optical phenomenon. The inexperienced can confuse aventurine with jade; however, even a novice can quickly learn to spot the disc-like inclusions in aventurine that distinguish it from jade or any other translucent green gem material.
Other minerals that can cause aventurescence in quartz are hematite, rutile and pyrite. In these cases, the body color will be either a reddish or brassy hue.
The name aventurine has an interesting root, as it borrows from a word previously ascribed to a manmade glass. Legend has it that an apprentice glassmaker in Italy accidentally spilled metal filings into his pot of molten glass. This chance happening, or “aventura,” produced such pleasing results that the spangled glass met with immediate favor. We know this glass today as goldstone.
Tiger’s eye, a brownish yellow chatoyant quartz, is one of the better known varieties. It commonly is used as an ornamental stone, especially for carving cameos and intaglios. Tiger’s eye is readily distinguished from other cat’s eye gems by its fibrous nature. It actually is formed when a multitude of small quartz crystals replaces a pre-existing mineral, but retains that mineral’s structure (called a pseudomorph in mineralogy – “pseudo” meaning false, and “morph” meaning form). The predecessor of tiger’s eye is a grayish blue asbestos mineral called crocidolite. Once replaced byquartz (silicified), the material most commonly oxidizes to a brownish yellow; however, it may remain unchanged in color. In that case, the grayish blue fibrous quartz is called hawk’s eye.
Tiger’s eye can be heated to a reddish brown, or dyed virtually any color.
The colored, crystalline varieties of quartz generally occur in pegmatites and veins. Many crystals achieve great size as a result of slow growth. The only color varieties that do not form in such large crystals are amethyst and rose.
If we could put a dot on every spot of a world map, it would be accurate since quartz is that universal. For the sake of space, we’ve limited our source list to a few notable producers.
Without a doubt, this country has been and remains the most prolific producer of quartz. All transparent (single crystal) varieties are found here – rock crystal, amethyst, citrine, ametrine, smoky, sagenite, cat’s eye, as well as rose quartz.
The amethyst found in various Brazilian deposits ranges from light ot dark. Some of the crystals have pronounced brownish or grayish color azoning, which can affect a cut stone’s overall purity or saturation. However, gem brownish Bart Curren notes that these brownish areas, when heated, turn a nice citrine color. As for size, Brazil produces many large crystals.
This South American neighbor of Brazil is known for producing very nice amethyst – a dark, rich purple color with the desirable red flashes. Crystals tend to be smaller than those from Brazil.
Bolivia and Argentina are other South American producers of quartz.
This African country produces highly saturated amethysts with a noticeable color change – from a bluish purple in daylight to a distinct reddish purple in incandescent. Zambian stones are very desirable and tough to come by, says Curren. Other African sources for quartz include Namibia (South West Africa), Nigeria, Zimbabwe, Tanzania and South Africa for tiger’s eye.
Other quartz sources of note include the United States (California, Arizona, Colorado, Texas, Montana, Arkansas, Virginia, North Carolina and Maine), Mexico, Canada, Japan, Madagascasr, Scotland, Switzerland, the U.S.S.R, Burma, India and Australia.
Color range: Colorless, white gray, grayish blue, yellow, orange, brown, purple, pink, green and black.
Chemical composition: SiO2 (silicon dioxide).
Crystallographic character: Trigonal system (in U.S., considered a subclass of the hexagonal system). Horizontal striations on the prism faces are common.
Hardness: 7 on the Mohs scale.
Toughness: Good (no distinct cleavage).
Degree of transparency: Transparent to opaque.
Possible phenomena: Asterism (rose); chatoyancy (cat’s eye, tiger’s eye and hawk’s eye); aventurescence (aventurine); change of color (amethyst); and iridescence (iris).
Refractive index: 1.554 to 1.553. Extremely constant.
Pleochroism: Amethyst – weak to strong purple and reddish purple. Citrine – very weak light yellow and very light yellow. Smoky – weak brown and reddish brown. Rose – moderate to strong light pink and colorless.
Optic character: Uniaxial positive. Due to a spiral crystallation, an unusual uniaxial optic figure is often observed in quartz – a so-called “bull’s-eye” (the figure has an open center). It is seen only in quartz.
Ultraviolet flourescence: Generally inert. Rose may flouresce a pale purple under shortwave.
Absorption spectra: In green aventurine, “chrome lines” are visible at about 650 and 680 nm. None of the other varieties exhibits a distinct absorption pattern.
Reaction to heat: Infusible to a jeweler’s torch. Heating often is used to produce a citrine color from amethyst. Heating also may render amethyst a grayish green, or possibly even colorless.
Reaction to acids: Attacked by hydrofluoric acid.
THE WONDERFUL INTERNAL WORLD
Jewelers generally agree that salespeople should focus strictly on the external beauty of a colored stone – its body color, luster, brilliance, cut, etc. Unlike diamonds, the microscope has no place in a colored stone sales presentation (except to substantiate the natural origin of an occasional stone).
But if you carry quartz in your store, you may want to dust off that scope. This species definitely is worth a closer look. Why?
First of all, quartz is found as a major component in many different rock types representing each of the three main classifications: igneous, metamorphic and sedimentary. And as a minor component, quartz is found everywhere.
John Koivula, GIA’s Chief Gemologist and renowned inclusion expert, explains the significance of its universal nature: “In view of the wide-ranging paragenesis of quartz, it should not be surprising that its inclusion family is vast.
Nowhere else in the mineral world is the diversity of nature so well displayed as in the world of inclusions in quartz. The longer one studies this subject, the longer the list of known inclusions becomes.” Indeed, Koivula himself has noted more than 100 different mineral inclusions in quartz, from albite to zircon. He calls quartz “our window to the earth’s interior,” since the growth and subsequent geologic history of a quartz crystal can be recorded by the inclusions it contains.
Which inclusions are of particular interest to collectors? “A piece of rock crystal with inclusions of gold, spessartine garnet or pyrite crystals is very desirable,” notes Koivula. “One thousand dollars is not an unusual asking price for such a specimen.” Quartz containing a phantom inclusion – characterized by a sprinkling of impurities that artistically marks the size and shape of the crystal at an earlier stage of growth – also finds a ready market.
AND A FEW SYNTHETICS
Like other natural gems, such as ruby, sapphire and emerald, quartz has a synthetic counterpart. But, unlike these other stones, quartz originally was synthesized commercially not to produce a more affordable and available version of “the real thing” but to supplement the diminishing supplies of clean material needed for technological applications. (Remember the piezoelectric property of quartz?)
Commercial hydrothermal synthesis of colorless quartz kicked off during World War II. It was just a matter of time before colored varieties appeared on the jewelry scene. Thus we now see synthetic amethyst, citrine, ametrine and even an “unnatural” transparent blue color on the market.
Identification of a synthetic gem material often poses a challenge to the jeweler/gemologist.
Various quartz crystals have been valued through the ages for their supposed magical, mystical or healing powers. But amethyst has the most colorful history and lore.
Greek legend attributes amethyst’s creation to Bacchus, the god of wine. Offended by some oversight by the goddess Diana, Bacchus declared that tigers would devour the first person he met. That proved to be the beautiful Amethyst on her way to worship at Diana’s shrine. In terror, Amethyst called to Diana to save her from the tigers. Before Bacchus’s eyes, the maiden turned to a pillar of sparkling white stone.
In regret, Bacchus poured the juice of the grape as a libation over the nowstone maiden, thus giving amethyst its brilliant violet color. Bacchus also promised that anyone who drank wine from an amethyst cup would not become inebriated. (Amethustos is the Greek word for “not drunk.”)
The ancients believed amethyst prevented violent passions, appetites and desires of the body. They also thought it insured peace of mind after conflict and temptation had been overcome.
In medieval times, amethyst was a soldier’s amulet, preserving the wearer in battle. Roman matrons valued the stone as a charm to retain their husbands’ affections. An 11th century German writer said a man’s amethyst attracted the love and affection of a noble women and protected him from thieves.
Amethyst has played a role in religion since ancient Egypt. It was the ninth stone in the breastplate of the High Priest of Israel and the stone of the tribe of Dan, standing for judgment. It has long been used in bishops’ rings; its color adorns the garments of church officials and kings.
Other quartz has been prized over the centuries. Thus, white quartz crystal supposedly discloses the future. Greeks reportedly discovered rock crystal quartz on Mount Olympus; they believed the gods created the crystals by permanently freezing water. Cutters have long fashioned rock crystal into spheres for future-gazing. Today, most such spheres are made of glass.