- Famous Opals
- Varieties of Opals
- Classification of Opals
- Formation of Opals
- Sources of Opals
- Opal Jewelry
- Physical & Optical Properties of Opal
- Test and Identification of Opal
- Valuation and price of Opals
The fever that lures men from the comforts of the cities to burrow for opals in the soft red rock of Australia's hinterland is a chronic malady - a fever that a man never gets out of his system. The opal miner is a solitary, silent, figure, forever in the grip of the fascination cast over him by the flashing rainbow colors of the gem the "cupid stone" to the ancient Romans who fell under its spell. His life is often one of hardship and privation. Bitter cold, intense heat and inadequate supplies and facilities are tolerated in the hope that his ceaseless efforts will eventually be rewarded.
What prompts man to search for opal under such adverse conditions? Why is it considered by many to be the most beautiful and desirable of all gems? The answer, of course, lies in the fact that its beauty is unique: the flashes of liquid fire that emerge from the heart of a fine opal produce an appearance that is found in no other gem, an appearance that has intrigued and mystified man since ancient times. The fiery display of colors is caused by light diffraction. The result is the familiar PLAY OF COLOR, the identifying characteristic of opal and its chief claim of fame.
Opal is an amorphous, hydrous form of silica. The amount of water present affects its R.I., as does any small amount of crystallization into one, of the several forms of quartz. X-ray studies have shown partial crystallization in some opals. If this occurs in layers, interference will result. Opal may well have accumulated in a pattern similar to agate and the layers may vary in water content or degree of crystallization, and thus in R.I., setting up an interference condition. As the silica gel lost water, it could crack, and these cracks might later have been filled with fresh opal of different water content and R.I.
The accepted theory today proposes that play of color is caused by diffraction of light and variations in refractive index from innumerable, regularly arranged, optically transparent spherical particles of amorphous silica and from the spaces, or voids, between these particles. The spheres, and hence the voids, are arranged regularly in three dimensions (face centered cubic), so that the whole arrangement makes a three dimensional diffraction grating. The important feature is that the spacing of the voids is the same as that of the spheres, and when this is about that of the wavelength of visible light, diffraction occurs. The angle through which the light is diffraction varies continuously with wavelength, so that different colors appear at different angles, thus producing play of color. Only pure spectral colors can arise from this process.
This theory of the internal structure of precious opal was proved in the mind 1960's by research with the electron microscope. Figure 1 shows the spheres under 24,000X magnification, the voids are illustrated in Figure 2 under 20,000X.
The historical background of opal is as colorful as the gem itself. There are numerous quotations and references attesting to the fact that has been admired and treasured by mankind for many centuries. The leaned Roman scholar, Pliny, described opal as having "the fire of the carbuncle, the brilliant purple of the amethyst, and the sea-green color of the emerald, all shining together in incredible union." The Romans called opal "cupid paederos" (child beautiful as love), and revered it as the symbol of hope and purity. At that time opals were beloved of God and man, and the stones were thought to preserve the wearer from disease. Eastern peoples placed the gem in a highly esteemed position, regarding it as a sacred stone embodying the spirit of truth. The opal, Orpheus wrote, "fills the hearts of the gods with delight". The Arabs believed that opals fell from heaven in flashes of lightening and thus acquired their marvelous colors. In ancient Greece they were supposed to possess the power of giving foresight and the light of prophecy to their owners. The Empress Josephine owned a gem that was called "The Burning of Troy", because of its dazzling play of colors.
Later in history, the praises of opal continued to be sung. The great Elizabethans were among its most ardent admirers. "This miracle was the queen of gems," wrote Shakespeare in "Twelfth Night." Marlow "Kew of Malta headed the inventory of his treasures with its name " Bags of fiery opals, sapphires and amethysts- jacinth, topaz and grass-green emeralds...." The artist, Du Ble, offered this poetic description "When nature had finished painting the flowers, coloring the rainbows, and dyeing the plummage of the birds, she swept the colors from her pallette and molded them into opals."
In the nineteenth century, a superstition was started that opal was an unlucky stone. This temporary eclipse in the history of opal's popularity was brought about when the eminent Sir Walter Scott wrote a novel called "Anne of Geirstein," in which the heroine owned an opal that mirrored her every mood, that flashed red fire when she was angry, and that "burned ashen gray" upon her death. Gradually, however, as public figures became known as opal fanciers or gave them as gifts, the superstition disappeared. For example, Queen Victoria gave each of her five daughters a magnificent opal. Sarah Bernhardt, the illustrarious French actress, was also instrumental in reestablishing the gem's popularity, for it was her birthstone and she never considered herself well dressed unless she wore opal set jewelry.
The name of the species comes from the Latin "opalus" from the Greek word "opalios", which itself comes from the Sanskrit word "upala" meaning precious stone.
A number of opals have become famous for their great size and beauty.
The Devonshire Opal, for example, is a magnificent black specimen from Australia. It is oval and slightly convex in shape, measures two inches long and one inch wide, and weighs approximately one hundred carats.
The Roebling Opal, a 2610 carat Nevada gem, is one of the largest masses of precious opal known, it is on display at the Smithsonian Institution, Washington, D.C. Also to be seen at the Smithsonian is the Roebling Black Opal, a 355.19-carat cabochon cut black opal from Nevada.
Another famous Australian stone, the Pandora Opal, weighs 711 carats, is four inches long, two inches wide, and nearly one inch deep. It is described as a good quality, light colored stone with a blood red pattern, interwoven with lightning flashes of brilliant bronze, gold and blue.
The Andamooka Opal, discovered in the Andamooka mines in South Australia, is a 205-carat oval shaped green gem with red, orange and blue play of color. It was mounted in a necklace and presented by the South Australian Government to the Queen of England in 1954.
Still another famous Australian stone is the 233-carat Flame Queen. It is a black opal with a flame-red center.
The Light of the World Opal is a fine 273-carat black specimen that was discovered in the early part of the present century in the Lightning Ridge district in Queensland, Australia.
One of the most spectacular finds, a 5 1/2-pound stone estimated to include about two pounds of finest quality opal, occurred at Andamooka in 1958.
Opal may be classified in any of several ways. The most logical for Jeweler's purposes considers basically the background color and transparency of gem varieties. In addition to the basic types, a number of terms are used to designate the dominance of various characteristics.
Opal is first classified according to the basic, or major, types as follows:-
Opal with play of color in a black or dark-gray body mass is called black opal. Those with dark blue, green, brown or other dark body colors are also called black opal, when the vivid play of color is also present. Less correctly, the term is used for stones with light-gray body colors.
Semi-translucent white stones showing play of color are called white opals, they are also referred to by some in the trade as "Hungarian opals", regardless of origin. The same people refer to any black opal as "Australian opal". Those with a greater feeling for accuracy speak today of Australian white or black opals. Fine qualities are sometimes called White Cliffs opal, after the Australian opal field of that name those with a milky-white body color are sometimes called milk opal.
Transparent to Semitransparent Opal
These have body colors ranging from colorless to yellow, orange, brown and red many have only a slight play of color, and some have none. The colorless, transparent stones are also called water opals or jelly opal. Transparent to semitransparent specimens with yellow, red, orange or brown body colors are known as fire opals, but some reserve this term for those without body color; such stones are usually faceted. All types in this classification, either with or without play of color, are often referred to as Mexican opal.
Included in this classification are all opals without play of color, with the exception of the aforementioned fire opal. They occur in every possible body color and vary from translucent to opaque. Very few are suitable for use as gemstones.
The second step in the classification procedure is to describe the varieties of black and white opal according to the size and shape of the color patches, the hue of the play of color, and the distribution of areas showing play of color, as follows:-
Classification of opals based on Size, Shape and Pattern of Color Patches.
Pinfire, or Pinpoint, Opal
Those in which the color patches are very small and close together. The small pinpoints can be of any color, but red is considered the most desirable. Figures 1 and 2 of the attached color photographs are examples of pinfire white and black opal.
Harlequin, or Mosaic, Opal
Those that exhibit play of color in rather regular, angular, close-set patches. Figures 3 and 10 of the attached color photographs are excellent examples of harlequin opal.
This term is used to describe those gems in which the color shoots across the stone in sweeping reddish bands or streak much like wind-blown flame. Figures 4 and 5 are flame opal.
Opals in which the play of color shows in sudden flashes that quickly disappear or suddenly change as the stone is moved. The flashes may be of any color, but red is considered the most desirable. Figure 6 is an example of flash opal. However, it is not possible to show the effect of the colors changing or disappearing, as in a photograph or when both the stone and the observer are stationary.
Classification of opals based on the Hue of Play of Color.
Those in which the interference colors resemble a peacock's feathers - blue, green and purple or any two of these colors.
A type in which the entire surface exhibits a yellow play of color.
A term used for Opal showing a green play of color only.
Classification of opals based on the Distribution of Areas Showing Play of color.
This type consists of flat, alternating layers of gem- quality opal with play of color and common opal without play of color.
Similar to onyx opal, except that the layers exhibiting play of color are irregular.
This variety is dark brown to black and opaque. Only small, isolated areas show play of color.
Cat's -Eye Opal
A variety in which the play of color produces a chatoyant band.
Dark brown and opaque, showing small patches of play of color and the structure of the replaced bone. It is sometimes called "Greek opal".
Matrix Opal, or Opaline.
Opal containing, or surrounded by, some of the rock in which it occurs.
Since none of the varieties of common opal can be considered as gemstones, they are described here merely as a matter of reference. None show play of color.
This type shows a moving, billowy light effect. It is almost transparent and the body color is milky white or very light tones of various hues. It resembles a poor-quality moonstone. The term girasol originated from the Latin words "girare"(turn) and "sole" (sun).
Translucent to opaque green common opal. The word is from the Greek "prasios", meaning "leek green".
A type that bears a close resemblance to moss agate.
A bluish-white, porous, translucent to opaque porcelain like variety. The name is adapted from "kaschtschilon," a word of Tarter origin.
Hyalite, or Muller's Glass
This type ranges from transparent and colorless to translucent and white. The name is derived from the Greek word meaning "glass".
Hydrophane, or "Magic Stone"
A kind of common opal that must be immersed in water or boiled in oil to produce a play of color. The name originated from a Greek word meaning "apparent in water".
Rose Opal, or Quinzite
A translucent to opaque pink variety.
Opaque and red to brown.
Liver Opal, or Melilite.
Opaque and gray to brown.
Transparent to opaque and yellow to light brown with a resinous appearance.
Opaque and red.
Opaque common opal of any color. The term is used frequently in the Australian opal fields.
A type of common opal that forms in the joints of bamboo.
A very porous type deposited by geysers.
In addition to all of the kinds of opals just discussed, six additional types must be considered
Small, broken pieces of gem opal placed in liquid-filled transparent glass containers.
Usually, sections of opal too thin for use as gems are cemented to pieces of chalcedony, glass or inferior opal, using a black adhesive. Such substitutes are often exceptionally lovely. A less common style consists of a thin piece of opal cemented into a shallow depression in a piece of black chalcedony. It provides an unusually attractive stone, since the black border contrasts sharply with the opal center. In cheaper stones of this kind, opal chips are cemented into the depression.
Transparent to translucent opal on which the back is painted black to improve the appearance of the stone.
A new assembled opal has recently appeared on the market. It consists of a base of common opal, chalcedony or other material, a thin layer of gem-quality opal and a top of colorless quartz. It resembles the opal doublet, except for the quartz top, which adds durability and prevents scratching of the softer opal.
Treated Black Opal
Very low-grade porous opal is often treated to darken the stone and give the appearance of black opal. It should not be confused with the rarely seen "baked" or "smoked" opal.
The stone is first soaked in a sugar solution, permitting the liquid to penetrate. Then, it is dried and immersed in a sulphuric acid solution that carbonzies (blackens) the sugar, creating a dark background for the play of color. The material used for this kind of treatment is a rather opaque opal in which the play of color is almost indistinguishable from its background.
A relatively new kind of black opal named oolitic, because of its appearance of being composed of a multitude of spherules, is reminiscent of oolitic limestone. In this stone, the play of color appears to act as a background for numerous black or brown spherical inclusions scattered throughout the stone.
Opal is formed at temperatures ranging from those prevailing at ground-water level to a few hundred degrees centigrade from silica bearing solutions. It may be deposited late in the cooling of igneous rocks, deposited during the alteration or weathering of silica bearing metamorphic and other rock types, or as a sedimentary deposit resulting from a concentration of silica gel in evaporating seas, often replacing shells or skeletal remains of sea animals.
In Australia it occurs in boulders of sandstone at depths of a few feet to several score of feet under the surface (BOULDER OPAL) in layers of varying thickness between common opal as a filling in cavities in sandstone, lava or other rocks or in limestone or clay. Gem-quality opal is usually found in sedimentary deposits in thin bands or seams (SEAM OPAL). Large stones are uncommon.
(Note: The numbers in parentheses in this portion of the text refer to approximate locations of sources on the accompanying maps).
Although opals are produced in many regions throughout the world, Australia is by far the most important source. The mining regions are located in New South Wales, South Australia and Queensland.
There are three general localities in New South Wales where opal has been mined extensively. The most famous of these is the Lightning Ridge district (1), where black opal was first mined commercially about 1905. This field lies approximately sixty miles north-northwest of Walgett (a total distance of about 500 miles from Sydney) and includes another important producer, the Grawin field, which was opened in 1926. At Lightning Ridge the opal occurs as seams in sandstone or as nodules in a sandy clay. Depending on the topography, the opal beds may outcrop or be as much as one hundred feet below the surface.
In the Tintenbar district, near Linsmore (2), opal occurs as loose fragments in the soil and in seams and veins in basalt.
The White Cliffs area (3), which is the oldest of the recognized fields in the industry, began production in 1889, shortly after a hunter picked up a fine specimen while trailing a wounded kangaroo. It is located in the county of Yungnulgra, about sixty miles north-northwest of Wilcannia. Here the gem opal is usually found as small veins in common opal. Both white and black material is produced in New South Wales.
The most important producing region in South Australia is doober Pedy (4), which is an aboriginal name meaning "white man in a hole", an allusion to the fact that almost everyone there lives in dugouts. This field, which is located in the Stuart Range, about seventy miles west of Anna Creek Station and 600 miles from Adelaide, was discovered in 1915, Only white opals are produced in this locality they occur in sandstone and claystone.
The most recent discovery that has led to the establishment of a field was in 1930, when opal was found at Andamooka Station (5), west of Lake Torrens. The combined output of Coober Pedy and Andamooka represents approximately 95% of the world's current production of gem-quality opal.
An interesting sidelight on the Coober Pedy field concerns its discovery and the living conditions of the miners and their families. A prospector and his son, in search of gold, decided to set off overland from South Australia to Western Australia. While camping one night in the Stuart Range, the son picked up a stone to throw at a camel. The boy was entranced as it curved through the air in a flashing, sparkling are toward the camel's back. He took the stone to his father, who showed it to a geologist. The expert told him that it was not a valuable find that it was what mining men called a "floater", but he indicated that it was an indication that there was valuable material in the vicinity.
So a party of six men and their camels started off. They lost their camels when only a few miles from the lake bed where they had decided to dig, so they began the rest of the journey on foot. They had not gone far when they met two men with a horse and cart. These men had discovered the Coober Pedy field, had worked six months for a profit of $80,000, and had decided to leave. So the first "gougers" (local term for opal miners) moved out and the incoming team moved in and the township of Coober Pedy was off to a flourishing start.
Today, Coober Pedy is one of the few places in the world that has no housing shortage, for as stated above, everyone lives in dugouts. In the boom years prior to World War II, the population was about 400, but the local citizens do not mind how many people want to rush there now. To construct a house it is merely necessary to go to work with a stout heart, a strong back, and a pick and shovel. You don't need bricks and you don't need timber; in fact, the furniture of most Coober Pedy homes is carved out of the walls.
The first dwellers chose to live in holes in the ground as a matter of necessity. The later inhabitants have stayed there out of preference, because below the surface they have excellent protection against the vagaries of the climate. In the summertime the thermometer rarely drops below one hundred degrees in the winter thick frosts cover the ground, and underground the temperature is always more tolerable. Most of the dugouts are equipped with cooking stoves and are fairly comfortable. A few have electric lighting, which is provided by storage batteries charged by wind-driven generators, and some even have kerosene refrigerators. Some miners bring their families to live on the field, but those with children try to move away during the worst of the summer.
The production from Queensland falls considerably below that of New South Wales and South Australia. Most of the deposits are close to the surface in sandstone or iron-bearing claystone. A large district in western Queensland is known to contain opals, but mining activity has been restricted to only a few localities, such as the Opalton field (6). These localities yield white opal. Black opal is known to occur in one area near the southern boundary, notably the Hayricks Mine, near Quilpie (7).
Prospecting and mining for opals in Australia is a hazardous and highly speculative venture. Not only are the deposits spread over a wide area, but there is little to guide the prospector to areas rich enough to pay. Surface indications, when they are found, consist of bits of weathered and disintegrated opal, which are called "colors", or "color floaters", and these indications are followed until the vein is located. Sometimes, however, prospectors merely dig at random and hope for a lucky discovery.
The mining itself is essentially a small-scale surface proposition; large-scale operations seldom prove remunerative. Usually, a pit is sunk until the opal-bearing stratum (called "hard band" or "steel band") is reached, and then the digging continues through to the soft layer (known as "opal dirt") that usually lies underneath. A tunnel is extended into the opal-bearing layer and the gems are removed from both the roof of the hard band and from the softer opal dirt. The valuable material is extracted from the hard material with a pocketknife, screwdriver or similar tool. Such trimming as is necessary is accomplished with a pair of pliers, and then the material is ready for sale as rough stones. Some of the Australian deposits lie very close to the surface and can be worked entirely by trenching methods.
The most important single factor that affects production is the climate. The principal fields are situated in isolated and arid regions, and even under the most favorable conditions the intense heat, scarcity of water and associated difficulties of the summer months induce many miners and their families to move nearer the coast for that period. If prolonged drought and other abnormal circumstances are superimposed on the naturally adverse conditions, the number of men leaving a field and the period of their absence will increase, and the production will show a marked decline. The effects of one or two drought years may extend over a much longer period, since some men will find other employment and will not return to opal mining.
The physical difficulties of prospecting in dry and inhospitable areas are very real, and prospectors must be well equipped to remain away from a supply base for more than a few days. Miners who are financially able to purchase good equipment naturally prefer to remain near the field where they have made money or to give up opal mining altogether, whereas others who would be prepared to go prospecting cannot afford the necessary equipment. Thus, very little of the money earned during a boom period is used in developmental work or further prospecting, and as soon as a rich patch is worked out there is a recession in the general level of production until another fortuitous discovery is made. Until some means are devised of overcoming this undesirable feature, it is unlikely that demand and production will be properly related.
Although a few lapidaries are engaged in cutting and polishing opals in the fields, principally for the tourist trade, most of the rough material is sold to buyers who visit the mines periodically, or to resident agents who act on behalf of city buyers. Business is transacted on a cash basis. Sydney is the center of the cutting and polishing industry, but lapidaries are also established in Melbourne, Brisbane and Adelaide.
Since boundaries were revised after World War I, the famous old "Hungarian" Mines now lie in Czechoslovakia. The center of mining is at Czerwenitza, east of Presov. Hungarian stones are well known to the trade, and for more than two centuries most of the fine-white opals came from this source. The gem material occurs in cavities in igneous rock, associated with common opal, pyrite and marcasite. Hot springs are still active in the vicinity. The discovery of opals in Australia reduced prices and finally, about 1920, resulted in virtual cessation of mining activity of these famous deposits. The Czerwenitza mines were worked so long and so thoroughly that they were practically worked out. Present production is negligible.
Mexico is noted for its fire opal and water opal, some specimens of which are very beautiful and desirable. The original source at Zimapan, in Hildalgo (1), is now virtually worked out. Fire opal is also found in the State of Queretaro (2), and in the State of Guerrero, near Huitzuco and San Nicholas (3). Other Mexican states, including Chihuahua, produce small quantities.
An unimportant variety of opal found in Mexico, described as opaque and creamy white, is sometimes "blaked" in an effort to improve its appearance. In this process, the fashioned stones are buried in an earth ware pot containing pulverized manure and opal dust from the cutting operation and heated approximately twenty minutes in a bed of hot charcoal. After cooling overnight, some or all of the stones will have turned black (as a result of the deposition of soot in the opal's "pores") and the play of color made more pronounced by contrast. The black coloration is only superficial, however, and the white color could be restored by polishing away the thin section that was blackened by the heating.
Some fine opals have been produced in the United States. The most important source is Virgin Valley, which is situated in Humbolt Co., Nevada, near the Oregon boundary (1). This deposit was discovered in 1908. A great variety of opals, ranging from almost colorless, transparent gems to the prized black type with vivid flashes of red, blue and green, have been found here. It is largely a desert region, where the opals occur associated with petrified wood in beds of volcanic ash. Sometimes the cracks of a piece of petrified wood are completely filled with flashes of red and blue. Occasionally, whole opalized twigs are found.
Unfortunately, these stones have a decided tendency to crack after being mined and exposed to the air, a condition apparently caused by the sudden loss of the excessive moisture that is a characteristic of Nevada opals. The deterioration usually begins with shrinkage of the surface of the stone as it loses moisture, producing a network of fine, shallow cracks on the surface. As drying continues, the cracks become deeper and the stone finally breaks. This condition is known as CHECKING or CRAZING.
Figures 1a shows partial checking; Figure 1b illustrates the appearance of a stone in which checking extends over the entire surface. If the checking marks are removed by repolishing, complete breakage may be avoided. This condition, of course, can occur in opals from any source, but it is a major problem with Nevada opals. To prevent breakage, the gems are sometimes buried for several months or years, in the belief that this will allow them to lose moisture slowly and to "temper" them. This attempt to "cure" opals is not always successful, however.
Another Nevada locality lies approximately fifty miles north of Winnemucca (2). The gems here also show a wide color range, but deterioration is an even greater problem than with those from Virgin Valley. A deposit of opal in basalt has been worked at Paradise Valley, Nevada, also.
Opal occurs as irregular veins in trachyte (an extrusive igneous rock) in the department of Gracias, especially near Gracias a Dios, Intibuca and Erandique. The material is similar to Australian white opal and often of good quality, but the deposits are somewhat inaccessible and little information concerning them is available.
Opal has been reported from a number of other localities throughout the world. Mention may be made here of its occurrence as small pieces of little value in an altered trachyte at Faratsiho in Madagascar, in rhyolite in the Tairna District on the Hauraki Peninsula in New Zealand, in the State of Carabobo in Venezuela, and from Tarakasaka and Hosakei in Japan.
Except for fire opal and water opal, which are sometimes faceted, all opals are cut in cabochon for jewelry, either flat, lentil or double. Seam type opal (Figure 2) can be fashioned by two different methods, the choice of which depends entirely on the nature of the matrix in which the seam is embedded. If the matrix material is sandstone, and thus too soft and friable to provide a suitable backing for the opal portion, the decision is to make a doublet.
This process is shown in Figure 3a. In the first illustration, the sandstone has been removed from one side of the seam. In the second illustration the exposed surface is ground flat and a piece of backing material (either chalcedony, glass or poor-quality opal) has been cemented to the flat portion of the seam. This cement, which is usually black pitch, serves two purposes: (1) it provides a dark background, which makes the play of color in the finished stone more noticeable, and (2) acts as a cushion for the thin piece of opal, because this type of adhesive never completely hardens.
In the third drawing, the remainder of the sandstone matrix has been removed. The fourth sketch shows that the cemented piece has been sawed in two and the excess trimmed off. In the final phase, the top and bottom portions have been cut and polished into finished opal doublets.
If the matrix consists of potch (common opal), instead of sandstone, the stone is cut with the matrix as a backing material (Figure 3b). In the first diagram, the matrix has been ground away from one side of the seam. In the next diagram, some of the excess potch has been sawed off. The third drawing shows that the piece has been sawed in two, and the last one illustrates the final result of cutting and polishing. The resultant stone is called Opal with Matrix, since the valuable portion extends to the girdle line. On the other hand, if a stone is fashioned from a piece of rough that consists of a small amount of opal and a preponderance of matrix, as in Figure 4, it is called Opal in Matrix.
Since opal may be damaged by heat or shock of any kind, great care must be:exercised in all phases of the fashioning operation. Sawing should be done with a very thin blade, keeping it cool at all times. Only fine-grit wheels should be used for grinding; not only does opal grind away very rapidly, but matrix may cause serious undercutting. Sanding is best accomplished with a worn sander or a very fine sanding cloth. For polishing cabochons, use a felt wheel with tin oxide or cerium oxide; either of these agents on a lucite or tin lap is satisfactory for faceted stones.
Since opal may be damaged by heat or shock of any kind, great care must be exercised in all phases of the fashioning operation. Sawing should be done with a very thin blade, keeping it cool at all times. Only fine-grit wheels should be used for grinding; not only does opal grind away very rapidly, but matrix may cause serious undercutting. Sanding is best accomplished with a worn sander or a very fine sanding cloth. For polishing cabochons, use a felt wheel with tin oxide or cerium oxide; either of these agents on a lucite or tin lap is satisfactory for faceted stones.
|Chemical Composition||Silica with varying percentage of water (usually 3% to 10% nearer the upper figure in fine quality), expressed by the formula SiO2 .nH2O|
|Crystallographic Character||Amorphous; however, some opals have proved to be at least partly crystalline.|
|Hardness||5 1/2 to 6 1/2|
|Toughness||Variable very Poor to fair. Opal is too fragile to be recommended for a man's ring. Although satisfactory for a woman's ring, it is necessary to warn an owner that care to avoid sharp blows is essential.|
|Specific Gravity||1.25 to 2.23 normal, 2.15|
|Streak||White to grayish|
|Characteristic Inclusions||Matrix consisting of sandstone, common opal (potch) or other materials.|
|Degree of Transparency||Transparent to opaque.|
|Luster||Polished surfaces are vitreous; fracture surfaces are sub-vitreous to waxy (the latter is more typical of common opal).|
|Refractive Index||1.42 to 1.47; usually, 1.45. Mexican opal may read as low as 1.37, but it is generally 1.42 to 1.43.|
|Optic Character||Singly refractive. Because of strain, transparent stones often exhibit anomalous double refraction|
|Phenomena||Play of color. Adularescence is sometimes exhibited 7 chatoyancy, rarely.|
|Ultraviolet Fluorescence||Black and white opal: moderate pale blue, green or yellow. Common opal and hyalite: moderate to strong green or yellowish green. Fire opal: very week to moderate greenish brown. All of these colors are produced by either long or short wavelengths.|
Effects Caused by:
|Heat||Sudden changes in temperature may cause opals to crack or fracture in addition, some are very porous. Thus, although it is common practice to re-polish the tops of worn cabochons while they are mounted, they should be removed for any other repair work. Opals should never be placed in a boiling solution nor cleaned in other than mild, soapy, lukewarm water. Overheating will turn most opals white or brownish and the play of color may disappear, in which case it cannot be restored.|
|Acids||Attacked rapidly by hydrofluoric acid.|
The appearance of gem opal displaying a fine play of color is so distinctive that sight identification usually suffices to distinguish it from other species. However, the unique appearance of the play of color does not mean that the entire stone is gem opal. Since the usual protective mounting covers the girdle of a flat opal cabochon, the possibility of a doublet must be considered, even though a fine play of color is evident. An un-mounted opal doublet shows clearly the line at which the top portion is cemented to the back (usually chalcedony or poor-quality opal) to strengthen the whole piece. The doublet is all but impossible to detect in a gypsy-style setting, and very difficult in any mounting that conceals the line at which the two pieces are cemented.
If the back of the mounting is open and a strong play of color is evident from inside the bezel, a doublet is unlikely, since the usual backing material is either chalcedony or low-grade opal. An absence of play of color through the back of the bezel suggests a doublet or a genuine opal, with fine gem material confined to a thin seam at the top of the stone. An opal in which the top portion is fine quality and the lower portion is patch is much less valuable than a stone that is fine quality from top to bottom. Doublets in which the base is common opal may sometimes be detected by using strong transmitted light, such as exists when the baffle is opened and the diaphragm stopped down in the Gemolite or Diamondscope. Separations in the cement, gas bubbles or arborescent designs will reveal the doublet. In certain cases, it may be necessary to un-mount the stone in order to identify a doublet positively.
The adhesive used to cement a thin seam of fine white or black opal to a poor grade of opal is almost always black. With a pitch or other black backdrop, the thicker, low grade white opal back is too opaque to be affected by the pitch. Thus, when a mounted stone appears to be black opal from the top and white from the base, it is probably a doublet.
Since gem opal is usually cut in cabochon, an experienced tester often has difficulty in obtaining refractive index readings, until he becomes adept at the "spot method". As a consequence, materials that could otherwise be distinguished from opal by R.I. must be separated by other means.
The only natural stone that bears any reasonable resemblance to gem opal is labradorite. Usually, labradorite exhibits an iridescence that is single hued over the entire top surface of the stone at any angle of observation. The color changes as the angle of view changes. To one who is familiar with gem opal, the resemblance is not close. In a labradorite rich rock, anorthosite, the individual grains are smaller and the rock is often black; therefore, the similarity to black opal is somewhat closer, but still far from identical. Anorthosite is used for decorative purposes on interior and exterior building facing, counter tops, etc. Any breaks that are visible on opal will be conchoidal fractures exhibiting a vitreous luster. Since labradorite has excellent cleavage, its breaks are usually flat. The low specific gravity of opal also serves to identify it.
Opal imitations have been made by sandwiching an iridescent metal foil between layers of glass that has not yet congealed and then contorting the glass to give the foil a broken, irregular surface. The imitation is so poor that it is easily detected by eye. Another imitation is formed by embedding fragments of mother-of-pearl in a clear plastic or glass. This, too, is readily detectable by eye for what it is.
With the exception of the problem of separating a doublet from genuine opal when mounted in a girdle-covering setting, the difficult opal identifications involve the varieties with little or no play of color. The term fire opal has been used to describe the transparent orange to orange-red variety, for which the term Mexican opal has come into common use in the jewelry industry. This could be very difficult to distinguish from glass, except for a distinct difference between the refractive index of glass and that encountered in gem opal. Usually, gem quality opal has an R.I. (1.41 - 1.45) that is somewhat lower than the lowest range of those encountered in the various types of glasses that are used for gem imitations. Transparent opal usually has an R.I. that is distinctly lower than most gem opal in the 1.37 to 1.43 range. Since transparent glass is not likely to have an index below 1.47, this is a significant difference and a satisfactory means of identification. Plastics such as bakelite may also resemble fire opal, but they too have distinctly higher refractive indices.
Treated black opal may be separated from natural black stones by using magnification and an overhead light source. Under magnification, the stone seems with black specks resembling black dust, resulting from dried residue of the coloring agent filling tiny cracks between gray or cloudy-white patches. Play of color appears in a mosaic pattern and seems to come from near the surface of the stone.
Oolitic opal is the only natural opal resembling the treated. Its dark body color is produced by brown or black spheroids surrounded by a lighter, ground or play of color. The circular nature of the inclusions, revealed under low magnification, is unlike the tiny black specks filling the cracks of treated opal and should cause no difficulty in distinguishing between the two.
An opal triplet presents no difficulty to the tester. Viewed from the side, un-mounted, the three separate layers are clearly visible. Even when mounted with the bezel completely enclosing the girdle, lack of play of color on the bottom, plus a quartz R.I. and transparency of the top portion when viewed from the side, will identify it.
Specific gravity tests are rarely necessary in opal identification. Although a diamond balance determination does no harm, heavy liquids could damage the relatively porous gem opal: thus, heavy-liquid tests should be avoided.
Semitransparent to opaque opal entirely without play of color is known as common opal. Some types of this material very closely resemble various chalcedony varieties, such as chrysoprase. Since opal may be partly crystallized, it sometimes appears light in the dark position of the polariscope and otherwise approaches chalcedony in properties. Since both are silica and chalcedony also has a water content, opal may be considered to grade into chalcedony as it gradually crystallizes. However, there seems to be no gem opal with properties overlapping those of chalcedony.
Three principal characteristics are used to establish the value of an opal:
- The intensity of the colors,
- The distribution of the colors and
- The number of colors present and their relative abundance.
An ideal stone is semi translucent, with the play of color consisting of individual patches of colors varying in hue, one from the other, and each large enough to be visible individually at normal arm's length.
Roughly, the order of desirability of color play in black and white opal is red, violet, orange, yellow, green and blue. On the other hand, in Mexican opals, violet, green and blue are the most sought for colors, since they contrast most strongly with the usual body colors.
The wholesale per carat value of black opal ranges from $50 to $750. Material with a play of color that consists primarily of blue and green flashes ranges from approximately $40 to $100 per carat, depending on the intensity and distribution of the colors. If desirable colors such as red, violet or orange are present, the opal is much more valuable. Stones in which the above colors are of rather low intensity and distribution only in a few areas are in the $100 to $200 per carat category, whereas those with a predominance of red and violet flashes of medium intensity are normally in the $250 to $350 area.
Opals with a variety of intense colors, including red and violet, usually bring between $400 and $600 per carat, depending on the size and shape of the stone, as well as the overall impression the stone makes. The prices mentioned above generally apply to opals between 3 and 15 carats. Larger sizes usually "soften" in price, unless the stone is exceptional in all respects.
Sizes below approximately 3 carats are considerably less. The wholesale per carat value of white opal ranges from $8 to $70. Fine stones in any size are rare. Unfortunately, the majority of stones on the market are rather gray or milky and, in the extreme, fall below $10 per carat. Matched stones (i.e. , those rather similar in appearance), either in pairs or in large numbers, may be offered at prices above those expected of single stones. Poor quality white opals cut to standard mounting sizes often cost as little as $4 to $20 per stone these show but little play of color, and only in streaks and small patches.
Better quality material, showing blue and green flashes, ranges from approximately $24 to $160 per carat, depending on the intensity and distribution of the colors. White opal in which the play of color includes red, violet or orange are in great demand and bring from $50 to $150 per carat. The $8 to $16 per carat range includes stones in which the colors lack intensity and are distributed only in a few areas, whereas those with bright colors spread completely across the stone are in the $24 to $400 per carat category. Stones that are exceptional in all respects, including size and shape, bring between $400 and $650 per carat. Very large stones are usually lower in price.
As a rule, the cross-section shape of an opal is of less importance when determining quality than with other phenomenal stones, such as star sapphires. Opals rarely occur in masses sufficiently thick to permit cutting high cabochons, and even then such shapes are usually avoided, since the maximum display of color is usually obtained when the surface is parallel to the platy areas producing the interference of light. In the better-quality opals these platy areas are usually in parallel or nearly parallel planes, thus, a high cabochon would reduce the beauty of such a stone. In addition, two flat cabochons of fine quality could be cut from a piece of rough that would yield a high cabochon. The girdle outline is much more important. Oval shapes are the most popular, although fancy shapes such as triangles and squares, if in pairs, rate equally with ovals of similar weights. The irregular shapes that are sometimes encountered are evaluated most easily by estimating the weight that could be recovered by re-cutting the stone to an oval or standard fancy shape.
In the interest of safety, attention must be given to the thickness of a solid opal. The shortage of the opal supply has been an incentive to sell opals that are much too thin for jewelry use.
Opal doublets are judged on the same basis as solid opals, except for the following considerations. They are most logically sold on a per stone basis, rather than on a per carat basis, since the backing usually weighs several times as much as the gem quality portion. The thickness of the gem portion is another factor in determining value. The price of opal doublets ranges from approximately $24 to $160 per stone, with the price determined by the same factors that govern the value of black and white opals, as described above. Those priced as high as $600 must necessarily have a thick gem-opal portion. A doublet with a very thin gem portion is quite fragile, and only a slight bump or pressure can cause it to crack.
Floating opal and stones made by cementing opal pieces into black chalcedony are inexpensive, usually costing from $10 to a maximum of $90 each.