Quartz Crystals: Clear and Colored Gems

Amethyst was reputed to protect one from drunkeness

Some Basics

Mineral Identification
Collecting Tools, and How to Use Them


Introduction to Geology
Our Changing Earth
The Geologic Time Scale
Stories Fossils Tell
Earthquakes and Faults
The Ouachita Mountains
Energy Resources: Fossil Fuels

Quartz Crystals

Introduction to Quartz
Digging Quartz Crystal
Cleaning Quartz Crystal
What's it Worth?
Types of Quartz
Geology and Mineralogy
Quartz as Gems
Experiments You Can Do

Other Collectable Minerals


Managing a Collection

Making Your Collection the Best
Cleaning Minerals
What to do...

Minerals Special to Arkansas

Some are New to Science


No Gold in Arkansas


Quartz as Gems

Colorless Quartz, also known as Rock Crystal

crystal ballCRYSTAL IS A TERM that began with the ancient Greeks to describe the colorless variety of the mineral quartz. They thought that water from deep in caves had frozen so solid that it would not thaw. The Medieval Japanese thought that rock crystal was ice or snow that was frozen hard. These beliefs probably originated from the fact that during this time much rock crystal was collected from veins that filled rock clefts in the mountains, where glaciers and permanent ice is seen. In the absence of modern science, we might think the same! In 1669, the great Danish physician, theologian, and geologist, Steno, made a startling discovery. By study and measurement of hundreds of quartz crystals from many locations, Steno discovered a law now known as the constancy of interfacial angles. This law is one of the fundamental tenants of the scientific study of crystals -- Crystallography.

Quartz is relatively abundant as colorless, transparent, uniform crystals of adequate size for lapidary uses. Quartz is somewhat brittle so it chips easily. The mineral has low dispersion and when faceted it produces little colored reflected light (we call fire), but it does have a high enough index of refraction that faceted stones are quite sparkly. Let's not get the crystal in stemware and chandeliers confused with quartz crystal. The former is a special formula of leaded glass that has a high index of refraction and is a man-made material, not a natural mineral.

The physical properties of quartz, combined with its relatively great abundance when compared to many other gem materials, allow it to be one of the most used raw gem minerals. Quartz is cut into beads and spheres, carved into many types of objects, from animals to abstract objects d'art. It is faceted into beads and variously shaped individual stones. Sometimes carving and faceting techniques are combined to produce positive- or negative-relief stones. Carving can be simple or very ornate. Another carving style produces cameos. Carving methods involve some type of abrasive powder, nowadays most often diamond imbedded in carving bits. Simple domed cabochons are often cut and applied as protective capping material for soft gemstones. This technique is commonly used with opal to produce "doublets".

History of lapidary use
Some examples of the use of quartz by lapidaries will illustrate how early man actually learned and perfected the lapidary trades. Although dating back to early Grecian times, the utilitarian use of carved rock crystal really became popular with the Romans. A list of items owned by Nero included wine jugs, cups, vases, and drinking cups carved from flawless quartz crystal. Lenses of rock crystal were used to concentrate the sun's rays to produce heat for cauterizing wounds and lighting fires, particularly sacrificial fires. In Mexico, a life-sized carving of flawless crystal in the form of a human skull was discovered in the Aztec ruins. It weighed almost 39 pounds. We don't know what religious use it had, if any. One of the Chinese emperors had colorless rock crystal doors to his throne room so that he might view his subjects while remaining warm in his heated area. But the art of quartz carving reached its zenith under the graces of Catherine the Great of Russia, who was a royal patron of this industry. Thousands of beautifully rendered art objects made from rock crystal and many other varieties of quartz were manufactured in old Russia. Perhaps the best known example in America is a seal showing Atlas holding the world. It was on display in the American Museum of National History in New York for many years. Generations of Chinese artisans have made lapidary contributions done through the centuries. The Chinese have specialized in exquisitely rendered miniature carvings made from colorless crystal, including vases, snuff bottles, ink boxes, incense burners, pendants, and figurines. Many other varieties of quartz have been utilized for the same purposes.

Crystal balls
However, no utilitarian object I've mentioned attracts more attention than flawless balls or spheres of quartz. Their manufacture dates far back into antiquity. The early Hindus, Greeks and Romans used small ones as reading balls (magnifiers). Small spheres are still quite popular today as gazing balls. Any flawless quartz sphere over 6 inches in diameter is usually out of reach, monetarily, of the average person. We know little of how the ancient Greek or Roman lapidaries made these reading balls. However, the extremely laborious methods of the ancient Japanese are well documented. The stone was roughly chipped to a spherical shape with hammer and chisel, then rolled in an iron trough with water and sand until perfectly round. Emery powder was used in a similar manner to produce a satin polish. Final polish was accomplished by the hand-rubbed application of rouge. In this manner, a finished sphere could be produced in a few years! Flawless quartz spheres are sometimes placed on display for all to view. At the Centennial Exposition in Philadelphia in 1876, a 7-inch diameter sphere from Japan was exhibited on a solitary stand. Around it were several cases of all types of ornaments carved of rock crystal. At this time a perfect 4 inch sphere from Japan might be purchased for $ 2,500. Another sphere, 6.5 inches in diameter, was on display at the Colombian Exposition in Chicago in 1893. It was valued at $ 10,000. Presently, the largest flawless quartz sphere in America is owned and displayed by the National Museum in the Smithsonian's Museum of Science and History -- Gem Hall, in Washington, D. C. It is 12 and 7/8ths inches in diameter and weighs 107 pounds. It was valued at around $ 250,000 in 1938! A trip to the Nation's Capital is not complete without visiting the National Museum and seeing both this sphere and the Hope diamond. They are displayed in the same room.

Inclusions in Rock Crystal
Inclusions of other minerals in rock crystal and smoky quartz are not particularly uncommon and are often highly attractive. They may be of two general types: early formed minerals that were captured by later formed quartz as it grew and minerals that were forming in or floating in the hydrothermal fluids that were deposited on the growing quartz crystal. Examples of the first type include rutile, actinolite, hedenbergite, some hematite, and tourmaline crystals that formed early and had rock crystal deposited over them, thereby encasing the early minerals. The latter type includes chlorite, cookeite, hematite, mica, pyrite, galena, adularia, and many others, that were deposited as the quartz was crystallizing. In some specific instances, such as asteriated quartz, rutile is thought to have been in solid solution in the quartz and, upon cooling, the rutile was forced out of the quartz structure, resulting in microscopic crystallographically oriented needles. The light, playing off these oriented needles, presents a six-ray star typical of asterism. The various types of included crystals give rise to a variety of lapidary materials and uses. Major sources for these types of gem materials are located in Brazil and Africa.


There are many books written that cover the various types of quartz, including rock crystal, but I think the following literature is some of the best around:

Quartz Family Minerals, a Handbook for the Mineral Collector by Dake, Feener, and Wilson, 1938, McGraw-Hill Book Company, Inc., 304 p.

Quartz crystal in Brazil by Johnson and Butler, 1946, GSA Bulletin, 57:601-49.

The properties of Silica by Sosman, 1927, New York, Chemical Catalog Company, Inc.,856p.

Quartz Crystal Deposits of Western Arkansas by Engel, 1952, US Geological Survey Bulletin 973-E, p. 173-258.

An Introduction to the Rock-forming Minerals by Deer, Howie, and Zussman, 1966, New York, John Wiley and Sons, Inc., 528 p.

Structure and vein quartz of the Ouachita Mountains of Oklahoma and Arkansas by Miser in The Geology of the Ouachita Mountains -- A Symposium , Dallas Geological Society and Ardmore Geological Society, p. 30-43.

Colored Varieties of Quartz gemstones

WE ARE GOING TO discuss the many varieties of quartz, other than rock crystal, used as gemstones and their geologic occurrence. These include the crystalline varieties amethyst, citrine, smoky, rose, adventurine, and the cryptocrystalline varieties agate, jasper, chalcedony, and others. We will also cover opal, even though it is not quartz, because it is a silica-based mineral that contains variable amounts of water.

Much of what was said in the previous article concerning rock crystal also applies to the other varieties of quartz. Since they are all the mineral quartz, no matter what the color or crystallinity, the statements concerning the physical properties of the mineral all continue to hold true. Each of the colored varieties of quartz varies considerably in distribution and geologic environment of formation from the other varieties.

We'll begin with amethyst. Amethyst is a purple variety of crystalline quartz. It owes its name to the ancient Greek word amesthusos, meaning not drunken. It was thought by the ancients that by wearing amethyst, you would not get drunk. Amethyst was a popular stone in ancient times, having been recovered by archeologists from burial crypts and ruins of early Egypt and Babylonia, seals and ornaments of adornment being commonly found. Beads of amethyst have even been recovered from early Anglo-Saxon grave sites in Britain. Amethyst was known to the writers of the Bible, being mentioned in Revelations 21:20. The fisherman's ring of the Pope and the rings given to Cardinals upon their investiture have amethyst stones. The source of amethyst for the ancients was most likely India and Sir Lanka (formerly Ceylon). During the Middle Ages, amethyst was commonly thought to protect its wearer from bodily harm in battle so it was often a stone attached to the Crusader's rosary. The oldest of all the Crown Jewels of England is a cut amethyst stone that was worn by Edward the Confessor, who became King of England in 1042. In the 18th century, amethyst was valued nearly as highly as diamond. A single necklace of Queen Charlotte of England was valued at $10,000. The discovery of vast deposits of amethyst in Brazil and Uruguay caused it to be a stone available to everyone. Vast lava flows with large gas cavities are the host rock for the Brazilian and Uruguayan deposits of amethyst. Amethyst crystals line agate geodes that may reach 4 to 5 feet in height and 2 to 4 feet across. Depending upon the size and quality of the individual crystals, these geodes may cost over $10,000. Most amethyst crystals display phantom zoning as color variation or patterns in the crystal. To produce high quality faceted stones, amethyst must be uniform in color. Although color is somewhat subjective to the buyer's preference, the deeper colored stones command a higher price as they are much more scarce. Varicolored and/or flawed amethyst is commonly used for carvings, spheres, and decorator items. Even amethyst geodes of various shapes are popular with US decorators at this time. Amethyst crystal specimens are well known from several regions of Mexico, and from Africa, Canada, Russia, and some states in the USA. Most of these deposits are in hydrothermal or pegmatitic veins. Amethyst occurs at only a few locations as elongate prisms like rock crystal, but normally forms short stubby crystals with six-sided terminations. The specimens from Mexico are exceptions to this rule, often crystals of amethyst from Guerrero and Veracruz form clusters of double terminated crystals, some individual crystals reaching 9 inches in length. The color of amethyst is due to the presence of iron, possibly linked to natural radiation.

Citrine is yellow to golden colored crystalline quartz. Here is a scarce natural gemstone that is readily available on the market as faceted stones at reasonable prices. This is because though few natural citrine crystals are collected, people long ago discovered that by taking weakly colored amethyst or smoky quartz and heat treating it, some small percentage would turn an attractive golden yellow. Russian peasants often placed these stones in their oven when baking bread. Much commercial citrine comes from Brazil, but it is probable that a large percentage of it is heat-treated. Much specimen material from Brazil consists of sections of geodes, having small citrine crystals with white bases and strongly colored tips. These were all most likely originally pale amethyst that has undergone heat treatment. As with amethyst, strength and uniformity of color are important factors in the pricing of cut stones. Natural citrine owes its color to the submicroscopic distribution of colloidal ferric hydroxide.

Ametrine is a trade name given to faceted stones that are both amethyst and citrine. When I first saw this material a few years ago, I didn't buy any cut stones because I thought it most likely was either manufactured or somehow heat-treated to obtain the result. The next year I saw gemstone rough, which consisted of sector-zoned quartz, alternating citrine and amethyst around the long axis of the crystal. It appears to be a natural material found only in Brazil. It makes most attractive transparent gems, the change of color taking place over a paper thin zone. Both the amethyst and citrine sectors are pale, but the color combination is quite pleasing. It is very reminiscent of bi-colored tourmaline, but much less expensive.

Smoky quartz is a variety of crystalline quartz that varies in color from light smoky yellow to very dark smoky brown. All gradations are seen. A nearly black variety is called morion. The color is often variable across the interior of the crystal, a detriment for use as faceting material. However, sufficient smoky quartz is recovered every year for large volumes to be cut. Flawed or varicolored pieces are used for carvings, spheres, snuff bottles, and objects d' art. The most famous deposits of smoky quartz are in the Swiss Alps. One well known Alpine pocket, discovered in 1867 and collected in 1868, yielded about 3,000 pounds of crystals. Many of these are preserved in the Berne Museum. The largest crystal, termed the King, stands 32 inches and is 3 feet around.

Nowadays, very large crystals as singles and clusters are being recovered from Russia. The best known American locality is the Pike's Peak region of Colorado, where some large crystals have been recovered from pegmatite veins. Other well known American localities include Washington State, North Carolina, and Maine, all sites of granite pegmatites. Brazil presently produces the largest volume of smoky quartz as lapidary enhanced pieces. Particularly attractive are polished crystals and spheres with inclusions of rutile, black tourmaline, or other needle-form minerals. In Arkansas, two areas have yielded smoky quartz specimens. The contact zone adjacent to Magnet Cove in Hot Spring County has produced some black smoky quartz crystals up to 15 inches in length and 6 inches diameter. These crystals are highly zoned and usually internally fractured, rarely producing a piece suitable for faceting. South of Blue Springs in Garland County is a mine, sometimes worked by Ron Coleman, called the Smoky Crystal mine. A single vein produces pale smoky specimens with stubby crystals up to 2.5 inches long. The quartz is lightly zoned and some material is suitable for faceting. Faceted smoky quartz is often sold under the jeweler's trade name ñ smoky topaz, although it is not related to topaz. It is a relatively inexpensive stone, but looks very good in gold. Consequently, it is usually sold in inexpensive gold-plated or gold-filled mounts. A large amount of smoky quartz seen at the various rock shops in Arkansas and elsewhere is rock crystal that has been radiation-treated to produce the smoky color. Most dealers will tell you right away that the material is irradiated, but sometimes you must press for an answer. If you see an entire table of black quartz from Arkansas having white to gray crystal bases, then more than likely it is irradiated. The color of natural smoky quartz is due to the presence of aluminum substituting for silicon in the lattice structure and natural irradiation processes which took place during or after crystal formation. Rock crystal that contains no aluminum substitution does not turn smoky by irradiation.

Rose quartz is a pink crystalline variety of quartz, rarely seen as crystals. Sites in the State of Mineas Gerias, Brazil, produced the most fabulous specimens of rose quartz crystals known. Groups and "flowers" of pale pink aggregate crystals to 6 inches across perched on milky quartz crystal are still available from some of the better dealers in Brazilian material. Tremendous amounts of rose quartz have been mined from one of the world's largest deposit in the Black Hills, some 8 miles from Custer, South Dakota. Rose quartz from this location may be seen at many rock shops across the country. Little faceting grade is encountered. However, high quality faceted rose quartz looks like faceted morganite, a much higher priced stone. The low-quality material is used for various types of ornamental work. The intermediate more translucent material is often carved in to bowls, animals, and sometimes cut into spheres. Other sites in America include deposits in Maine which yielded flawless carving pieces to 5 inches square. Numerous small bowls, dishes, and spheres were cut from the Maine material. Some rose quartz has been discovered in Connecticut, New York, and California. Recently, some small spheres of rose quartz displaying asterism as a six-rayed stars have been available from Mineas Gerias, Brazil. Asterism in rose quartz is due to minute oriented rutile needles that exsolved after the rose quartz crystallized. The color of rose quartz is due to traces of manganese along with traces of titanium and lithium.

Adventurine is a form of crystalline quartz that contains inclusions as bright scales of some mineral, usually mica or hematite, which presents a spangled appearance when viewed as the specimen is rotated. The stone is most attractive when cut cabochon-style and the included flakes are medium-sized. Generally, adventurine is not seen on the market as crystals, but as finished objects, including gem boxes, carvings, and snuff bottles. The quartz may vary in color, being brownish, red, yellow, black, and as a most desirable green color. The classic deposits are in the Ural and Altai Mountains of the old USSR. Bowls and other utilitarian objects were manufactured from adventurine from the Altai deposits. India has some deposits of this material also, but generally of a much inferior quality. Likewise, concerning some small deposits in Europe and the USA.

Cryptocrystalline varieties of quartz that have important lapidary applications include agate, jasper, chalcedony, chrysoprase, carnelian, and bloodstone. Cryptocrystalline quartz consists of submicroscopic grains of quartz. When examined by the electron microscope, we find the texture to be fibrous in most instances. The fine-grained varieties are generally deposited as late void-filling minerals from warm to cold ground water. Silica-rich waters percolating through fractures, solution cavities, or fracture zones deposit layered, banded, or uniformly colored fine-grained silica which hardens to make the different varieties.

Agate is the most popular variety of cryptocrystalline quartz among lapidaries. It is also the most abundant. Agate always exhibits banding. The range of colors and patterns appear limitless. How agate forms has long been a source of speculation. Present thinking involves the deposition of silica gel and the diffusion of color in it before it hardens. Several, if not hundreds, of separate layers of silica gel are deposited. Often, at the end of agate deposition, rock crystal or amethyst may be present as the final infilling. Some agates forming this way fill completely solid and some are hollow, depending on the continued availability of fluids rich in silica. However, examples of agate exist that show features which indicate that silica gel deposition was not the mode of formation. Instead the agate was formed by direct precipitation from percolating solutions. Because agate is so widespread, it seems reasonable to assume that it may have several different modes of origin.

Agate from some localities is so distinctive that just seeing the specimen is enough to determine the location. Examples are Mexican crazy lace, Brazilian, and Lake Superior and Laguna fortification agates. There are so many well known deposits worldwide it is beyond the scope of this class to adequately describe them all. Almost every state in the USA has some type of agate, whether as primary deposits or in gravels. Agate is normally cut in cabochons of many different shapes and set in inexpensive jewelry, although some moss agates and other higher priced unusual patterned stones may be set in sterling silver or gold-plated mountings. The term "aggie" marble refers originally to marbles that were made from real banded agate. Large agates from Brazil are commonly dyed or colorized and are seen as decorator items, bookends, and agate boxes. Much petrified wood is composed of agate. Some of this wood is highly colorful, like the Arizona petrified forest wood, and some is bland in color, but preserves the original wood's texture on a microscopic basis. This type of preservation is the result of replacement of the organic material of the wood on a microscopic level by silica. A type of lapidary material, called tiger's eye, also resulted from the same type of silica replacement process, but instead of wood, chrysotile asbestos was replaced by silica.

Jasper is a red to reddish brown to tan, bluish or even black, opaque cryptocrystalline variety of quartz that takes a high polish. The variation in color is due to dispersed iron oxide, the jasper originating, in those instances, as a silica gel. Generally not banded, jasper may have a large variety of patterns. Jasper is a relatively common silica mineral in many base-metal ore deposits, especially where silicification of the host rock preceded sulfide mineralization. Some varieties of jasper may be named after the location of the mineral, examples being catalinaite (Santa Catalina Island, CA), oregonite (Oregon), Sioux Falls (South Dakota), and Texas "agate" (Pecos River area of Texas). Other varieties are named for the pattern of the stone: orbicular, banded, jasper breccia, moss, variegated, poppy stone, and puddingstone. Puddingstone jasper is a jasperized conglomerate. All these sub-variety names lead to considerable confusion as to what the material actually is. During the Middle Ages in Europe, jasper was believed to be a very powerful medicinal stone, which if worn, warded off all types of ailments and problems. So it's not unusual to learn that it was a very popular stone, worn in all manner as personal adornments. Today certain types or patterns of jasper are still popular, worn as cabochons or carved as animals or novelty items. You may also see spheres of various sizes made from the more brightly colored or highly textured varieties of jasper.

Chalcedony is an essentially colorless to pale whitish to bluish gray variety of cryptocrystalline quartz. Jasper and agate, though highly colored and/or banded, have been grouped as types of chalcedony by some writers. All three share some common characteristics, including a fibrous submicroscopic texture with pore space between the fibers. The presence of the pore space allows the colorizing of agates and pale stones by dyes which selectively penetrate the stone via the pores. Much chalcedony is dyed green to simulate chrysoprase. Chalcedony, however, is typically fluorescent pale green to moderate green in ultraviolet light unlike jasper or agate. This is thought to be to traces of uranium salts. Chalcedony's uniform pale color, translucency, and ability to take a high polish make it a useful lapidary medium. It is commonly seen made into small spheres or carved as small animals.

Chrysoprase is an unbanded translucent yellow- to blue-green variety of cryptocrystalline quartz, the best grade being apple-green. It takes a high polish and cuts to make beautiful translucent stones, rivaling the best of the oriental jades. The green color is due to the presence of about 1 % nickel silicate included as a stain in the silica. This stone has been known and valued for centuries as evidenced by the so-called talismanic powers referenced to this stone by the ancient Egyptians, Greeks, and Assyrians. Chrysoprase has never been produced in enough quantity to meet market demands. Much of what is now marketed as chrysoprase is chalcedony dyed with nickel salts. For the past two years, some quantity of chrysoprase carvings on a tan matrix rock from China have been on the market. The prices are so reasonable as to make me concerned that the material might be carved, dyed chalcedony and the matrix rock is assembled with organic resins and then attached to the carvings and finished out. Excellent gem-grade material has been produced from Tulare County, CA, and scattered other sites in the USA. The bulk of the early supplies of this material was from the vicinity of Frankenstein in Silesia (modern Poland). Some other production came from the Ural Mountains.

Carnelian is a translucent to transparent orange to reddish variety of cryptocrystalline quartz, grouped by some workers as a type of uniformly colored agate. This material was widely used in the past as an impression stone for wax seals, hot wax not adhering to the polished stone. Deep rich colors in this stone are most highly prized, leading some individuals to carefully cook agate or chalcedony to produce the carnelian's color. This stone is mentioned in the Bible as being one of the gems in the High Priest's breast-plate. This stone has not been found as deposits of in-place veins or infillings, but instead is found on the surface of gravel deposits. This fact leads to the theory that this material is formed on the surface of the ground when chalcedony or agate are exposed to ultraviolet rays for long periods of time. With time, specimens deepen in color and depth of color penetration. Carnelian is found at many localities around the world and surface deposits, but the greater amount of higher quality material comes from desert regions, particularly in northern Africa. The western states of the USA, which include California, Oregon, Washington, Nevada, and Idaho, produce high-quality gem-grade carnelian. However, the world demand is not met by the present natural supply. This has led to the manufacture of much commercial material by the above mentioned method.

Bloodstone or Heliotrope (the older name) is a cryptocrystalline variety of quartz characterized by a translucent, dark green matrix of stone called plasma with scattered spots of bright, blood-red jasper. It takes a high polish. The green color is due to chlorite and the red to hematite. This stone was held in high esteem by all ancient cultures that knew of it and was extensively used for seals, signets in rings, and for decorative items.

The early Christian Church frequently commissioned sacred objects carved of bloodstone. Several carvings of the head of Christ were executed, one using the red spots to very realistically resemble drops of blood. The ancient tradition of the origin of this stone was that it was a green stone at the foot of the Cross, the red spots representing blood shed by Christ. In the Middle Ages, great curative powers were given this stone, both when worn and taken internally as powders. Even though bloodstone may be present anywhere jasper occurs, almost the entire world's supply is obtained from India on the Kathiawar Peninsula, near the sites of the agate, carnelian, and chalcedony deposits in the Deccan traprock, a series of massive basaltic lava flows. In the USA, some good quality bloodstone has been recovered from Newport Beach, Oregon and at a couple of sites in Georgia and New York. There is a large amount of mixed red and green jasper available that is sometimes called bloodstone, but the spots of red do not resemble drops of blood and the overall look is not as pleasing. Small spheres and cabochons are the most common lapidary use of this material, although some small carved animals have been available the past few years.

Opal was first thought to be an amorphous variety of silica that contained variable amounts of water. However, further mineralogical investigations revealed it to be a hydrous cryptocrystalline form of cristobalite with submicroscopic pores containing water. It may be any of a wide variety of colors. Opal may be divided into two great groups: precious and common types. It has been reported that there are some 52 varieties of opal. Precious opal displays a play of internal colors whereas common varieties show no internal colors, but often have a pearly or opalescent luster. Opal has the chemical formula SiO2 . nH2O, with the water content ranging around 6 to 10 percent in precious opal. The structure of precious opal has been shown to consist of a close-packed array of regular silica spheres. These spheres form a 3-dimensional diffraction grating, which is responsible for the play of colors. The pattern of the fire in precious opal are termed according to the internal patterns: patch, pin-fire, ribbon, and harlequin, to name a few. Some varieties of common and precious opal are named for the type of matrix , location, pattern, or color. These types include jelly opal, black opal, boulder opal, hyaline, milk opal, and moss opal.

There are only a few major locations that have yielded significant quantities of precious opal. The oldest opal fields in the world are located in Czechoslovakia, having been worked as early as the early Roman Empire. The opal from these mines fills fissures in a weathered andesite lava. Mexican opals occur as fillings in spherulitic rhyolite, an extrusive lava equivalent to granite. Mexican precious opal, termed matrix opal today, is mined in Queretaro, Hidalgo, Michoacan, Jalisco, San Luis Potosi, and western Chihuahua States. The best known precious opal now comes from Australia, in the state of New South Wales, northward into Queensland. Several major fields were discovered in the latter half of the 19th century and early part of the 20th century. White Cliffs, Lightning Ridge, Cooper Pedy, and Stewart's Ridge are well known fields. There are two geological occurrences of precious opal in Australia. One is in an Upper Cretaceous sandstone and the second is in a vesicular basalt of post-Cretaceous age. The opal rarely reaches 0.5 inches in thickness. Very interesting specimens of opalized fossils have been recovered, including bones, shells, and wood that have been transformed into masses of fiery precious opal. The opal fields of Australia are located in a desert environment. Mining has gone underground, with the miners living in the underground passages. A few years ago, unusually heavy rains flooded much of the underground workings and production has not recovered to pre-flood volume yet. In Honduras, some precious opal has been recovered of the same quality as the Australian material.

The USA has several precious opal-producing areas. Perhaps the most famous single opal ever found in the USA was recovered in 1919 from Virgin Valley, Humboldt County, Nevada. It weighed 17 ounces and was purchased for an estimated $125,000 by Colonel Roubling. It is a mass as black as a lump of coal, throwing forth brilliant and deep flashes of red, green, and purple fire. It has never been cut and is in the U. S. National Museum (Smithsonian Institute) collection. The opal of Virgin Valley is present in similar geological conditions as the deposits of Australia. A volcanic ash layer in the bottom of a Tertiary lake bed contains opalized casts or pseudomorphs of wood - trunks, branches, even conifer cones are completely replaced. In the state of Louisiana, precious opal was discovered to cement sands of a Tertiary-age unit. The color plays are mostly in greens and blues, but occasional red fire is also seen. Although insignificant compared to other opal sources, the Louisiana material is interesting as it shows the secondary nature of the opal.

Precious opal has some problems as a gemstone material. Because it contains so much water, it may dry out over time if not in contact with the wearer's skin. The oils it picks up from the skin help prevent the loss of fire. Indeed, most uncut opal is kept in oil, glycerin, or water, to prevent it from drying out. It is soft for a gemstone, only about 5.5 on the Moh's scale. It also is commonly only a paper-thin coating. Consequently, lapidaries perfected techniques to make up for these deficiencies. A colorless domed cap of quartz is cut and polished, then cemented onto the opal, and the matrix cut to remove the excess. This produces an opal termed a doublet, and has several effects. First, the dome of quartz acts as a magnifying lens, enlarging the flashes of fire in the stone. It also provides a hard cap and is an effective moisture barrier, which prevents the opal from drying out. Triplets may also be made if the matrix is too porous or crumbly. Black onyx or even black novaculite has been used for the backing stone. Black is a favored color for triplet backing because it enhances the intensity of the fire. The water content of opal causes the stone to be quite heat sensitive. Too cold and the material freezes and shatters, too warm and it dries out. Opal has a conchoidal fracture and is quite brittle so it chips easily. Opal has a reputation as an unlucky stone, unless someone else buys it and gives it to you. This probably came about due to the various problems this mineral has as a gemstone material.

With all the problems with this gem material, it requires some thought as to what type of jewelry it may be used in and the style of setting to protect the stone. I do not recommend the wearing of opal in rings. The hands take much abuse, consequently the ring stones do also. Pendants, necklaces, and pins and brooches are much more likely candidates for opal stones. Most precious opal is prepared as cabochons, either in standard sizes or as free-formed stones. Much precious opal is carved in the orient. Small opal chips are used for inlay work. The Chinese are known for carving very ornate small relief sculptures.

Common opal is not so popular as agate as a lapidary medium, but some Mexican deep orange jelly opal has seen considerable success as faceted stones. It has no fire, but the strong orange color is rare in nature. In color, it rivals the best Imperial topaz. Common opal is aptly named as it is relatively common mineral replacing wood and as fracture fillings in recently mineralized petrified wood. Common opal is found in many colors, typically white, black, gray, bluish, or colorless. Most common opal shares a rather unique characteristic of fluorescing some degree of green under the black light, due to traces of uranium salts. Some varieties do not, but they appear to the exception, not the rule. Common opal is generally found to be the last deposited mineral when it is found as it coats everything present. A few unique specimens have been found that appear to be colorless hyaline opal coating spider webs or filaments in open cavities. Essentially all the petrified wood in central Washington state is opalized. It takes a high polish and has pleasing patterns.

Virgin Valley, Nevada, is the premier locality for large masses of uniform pale green common opal that has a bright green fluorescence. Common opal with a highly cellular structure is often deposited from thermal or cold springs. It is sometimes confused with pumice as it will float on wood just like wood. Common opal in Arkansas has been reported from several different types of deposits. A siliceous replacement in the contact metamorphic zone at Potash Sulphur Springs in Garland County is an unusual geologic environment. One area that has produced colorless, though highly fluorescent, films of a variety of opal termed hyaline is near Spruce Pine, North Carolina. Late fractures are filled with hyaline in several feldspar pegmatites in the area. Specimens are best collected at night with a portable backlight on the mine dumps.