precious opal

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Precious opal is one of the more popular gems and is certainly one of the more colorful natural stones used in jewelry and carvings. While ruby, sapphire, emerald, and other gems are satisfied with being monochromatic, precious opal manages to capture and lash every7 color of the rainbow in an assortment of unpredictable patters that everyone. It is used in every form of jewelry including rings, even though its hardness is suspect at Mohs 5 to 6.5. It can be used by itself as a vibrant pendant or colorful ring stone or it can be used to complement the many monochromatic semiprecious gems used in jewelry and intarsia, like blue lapis, turquoise, red rhodonite, varicolored agates, brown or red jasper, and more. Thin slices of precious opal are very popular and frequently used by intarsia artists, who layer it in parallel strips with other gems or inlay it in geometric patterns.

Pliny the Elder was a Roman historian who wrote some of the earliest comments about precious opal. The opal he saw probably came from ancient digs in the Carpathian Mountains in what is now the Slovak Republic. With great admiration, Pliny wrote that opal had “all varieties of colors brought together to form an improbably picturesque blend.”

Precious opal can flash every color of the rainbow as single points of colorful light, bright spots, and sparkling lines, ribbons or streaks. When you rotate the gem in a strong light, the colors wink on and off, sparkle then fade. The gem is really quite unpredictable.

The most exciting opals are those whose groundmass (the part with no color) is transparent, or nearly so, as the color is best seen without interference. If the groundmass of the opal is cloudy, it can cull or inhibit the colors. The so-called black opals, with darker backgrounds, are really flashy and bright because of the dark, contrasting mass. When an opal that has good fire but a light background, a lapidary can make it a doublet, using a black material like onyx as a backing. That helps the opal’s color show to best advantage. The groundmass can even be treated to help the opal’s color.

Pliny wasn’t the only fellow smitten by opal. The noted author James Joyce compared opal to his wife to demonstrate his love for her. He called the unpredictable, fascinating but fleeting vibrant colors we see in fine opal “strange, uncertain lines and colors, warm lights and quick shadows.”

So how does all the color come from a lump of amorphous hydrous silica: After all, there’s hyalite opal, a clear, bubbly, amorphous silica that has no color at all save that it often fluoresces a brilliant green when it contains the chromophore uranium in it. And there is certainly more noncolorful common opal than the fiery and precious kind. Obviously, something unusual has to be going on in the opal to create color flashes in such random var4ieties and scattered ways.

The early belief was that opal was a magical stone that might contain good or evil. Later, that graduated into the strong belief that if you wore an opal and it cracked it was a very bad omen. Sir Walter Scott took that superstition one step further in his novel Anne of Gieistein. Anne had an opal, and every time she wore the thing it caused her grief. So in the 1700’s and 1800’s, opal, which had been one of the most exciting and admired gems, suddenly became a pariah.

I have to wonder is Scott had been embarrassed or mistreated by an opal dealer or some lady who owned a noteworthy opal. Scott was known to be vindictive. He was not averse to using his so-called fictional novels to get even with the people he disliked. For example, when Philip Rashleigh, one of the premier mineral collectors in Cornwall, England, in the late 1700’s, refused Scott’s request to visit and see his fine collection, Scott wrote Rob Roy, in which the evil brother was named Rashleigh Osbondestone! Rashleigh had the last laugh, however. It seems he died before Scott’s book was published!

People in those days did not understand that opal cracked naturally, and that the cracking had nothing to do with good or evil. Today, we have no such silly notions about fine opal. It is a very popular and frequently used gemstone. The cracking occurs as a result of water loss. There are spaces within the precious opal that hold water, which con be lost over time, since it is not tightly bonded to the silica. Loss of water increases the stresses in the opal, sometimes leading to cracking and crazing, myriad nearly invisible cracks that effectively damage the gem. The trick with opal is to wear it. Instead of putting opal in a safe place where it will ultimately dehydrate, you are better off wearing it, bringing it into contact with skin oils, which tend to seal the stone. The oils may dull the polish, but wearing the stone slows the dehydration process.

As scientists began to try to understand color in gems, including opal, they thought precious opal’s play of colors was due to internal fracturing or random microbanding from shrinkage. They were relying on what they saw around them. When oil contacts water, a rainbow of colors is seen. The same is true along the edges of broken glass. The rainbow of colors seen in these two cases is due to interference of light, which is not why opal has such lovely color patterns.

Interference of light happens when light strikes an apparently flat but slightly edge-shaped surface. Oil, as it spreads over water actually is not evenly spread. The edge nearest the source of the oil is microscopically thicker than the outer edge, so the effect is a wedge shape. When light strikes the oil, it is spread out according to the different wavelengths of color that make up white light, so you see a rainbow effect. The same thing happens with broken glass.

Now, we know the play of color in opals, when it does occur, is due to the unique internal structure of opal and not interference of light. You can argue the point, is but the mechanism is really entirely different, as you’ll see. It was not until the electron microscope was developed that we were able to look into opal’s interior. This happened in Australia in 1964. What scientists saw was an orderly linear and layered arrangement of tiny microgranules, or spherules, of silica. In most cases, these granules are cristobalite, a form of silica. In some cases, they are a diff4erent form of silica. In all cases, it is the orderly arrangement of these spherules that is necessary for light to be scattered to create opal’s colors. This orderly arrangement of globules has the ability to split light, absorbing some wavelengths and releasing others, so one or more of light’s various colors are release for you to see. This orderly arrangement of spherules, as long as they fall within certain size parameters, has been to a diffraction grating, which also splits light into its component colors. The arrangement has to be orderly, for when it is not, common opal is the result.

Scientists have now studied opal sufficiently to be able to relate the color flash you see to the size of the spheres responsible. In brief, the smaller the spheres, the shorter the wavelength of light you’ll see. Scientists have now shown that spherules that measure between 2,000 and 3,000 angstroms can create a desired color. If the spherules are below and above those sizes, the opal is unresponsive to light.

The smallest spherules within that range can result in a color that is a blue or violet flash. As the spherules increase in size (commonly the case within a single piece of opal, or every opal would only show one color), the next color you see is green, which grades into yellow as the sphere size increases again. The next larger spherules give us the lovely orange color we see, and the maximum size spherules that will still give color produce red flashes. Any spherules that are smaller than 2,00 angstroms or larger than 3,000 angstroms simply can not diffract light sufficiently to give a visible color. We call the unresponsive material common opal.

Just how these tiny spheres are able to develop in uniform size and arrange themselves in orderly rows depends on the conditions during opal’s birth. Weathering is the midwife that carries dissolved silica, usually absorbed from volcanic ash and silica-rich rocks like rhyolite, into seams, cracks and horizons of rock, in which the silica-laden water can collect and settle. The silica can then develop into tiny solid spherules ever so slowly. As they develop, they gather layer upon layer, row upon row, with clusters tilting this way and that, but always in orderly clusters. This is the first and most important part of the cause of color in opal.

In between these tiny spheres, water or even more commonly cristobalite can be trapped. The water might later escape, thus spoiling what was a color-producing arrangement.. The cristobalite is less likely to leave the structure. While the spherules are now formed, the medium in which they reside plays a role in developing color. That is the second important factor necessary for color. The medium – water, silica or some other dissolved mineral – that is between the spherules is important to the final color result.

It is the type or condition of what is called the interspherule space that affects the transparency of the gem, as well as the intensity of the play of colors. Sometimes, the spaces are filled with cristobalite, the same form of the quartz silica of which the spherules are made. The refractive index of water very closely matches the composition of the spherules. The resulting opal is what we call contra luz. This type of opal shows a most marvelous color effect: broad, flowing ribbons of flashing color, usually red or orange. They are among the broadest and largest color bands seen in opal. They occur because the refractive indexes of two forms of cristobalite are so close as to complement each other.

The better contra luz opal is Mexican fire opal dug from the rhyolites in the state of Queretaro, where it occurs as small, rounded masses that often fill gas pockets in the volcanic rock. The ground-mass color of these opals can be reddish, yellow or orange, caused by the close refractive indexes of the spherules and the interstitial space material. The better pieces of contra luz show those flowing bands of colors so bright that the opal is called fire opal or cherry opal. The best of this type of opal is from Mexico.

When the material in the interstitial spaces is not cristobalite, it is usually water. With a different refractive index, water will cloud, or at least slightly dull, the color you see. The same is true of any other material included in the spaces between the cristobalite spherules.

The world’s major opal supplier is Australia. It has a virtual stranglehold on the market. Fine opal is found in Brazil, the Slovak Republic, Russia, and Honduras, which was probably the source of the opals used by the Aztecs. Here in the United States, we have several productive opal sources: Virgin Valley, Nevada; Spencer, Idaho; Opal Butte and elsewhere in Oregon; and opal Mountain and elsewhere in California. Fee digging is possible in some of these places.

The premier precious opal source is Virgin Valley. Fortunately, fee digging is available as several mines should you decide to dig your own opal. The area is a bit remote, but well worth the effort to get to. Unlike opal from many other sources where the gem as formed in narrow seams within hard-to-crack rhyolite or other rock, the Virgin Valley gems occur as replacements of organic material, primarily wood.

The deposit is the result of volcanic action that spewed tons and tons of ash, which formed in layers to 1,000 feet thick. Trapped in that ash were fragments of wood and other debris, some of it charred by the hot ash, which eventually developed into a solid volcanic tuff, a compact, fine-grained material that is a mix of tuffite and montmorillonite clay. The wood gradually fossilized, but unlike the Petrified Forest of Arizona, this wood was not replaced by colorful agates. Instead, weathering carried dissolved silica from the volcanic ash into the spaces left as the wood slowly rotted. The exchange of opal for wood was so gradual that even the cell structure of the wood was preserved. As this happened, the silica spheres formed in ideal sizes and perfectly orderly patterns. The result is some of the brightest, most colorful opal found anywhere.

The three main mines in Virgin Valley are the Royal Peacock, the Bonanza and the Rainbow Ridge. Fee digging is allowed at a couple of these mines during the summer months. Contact the Rainbow Ridge Opal Mine at P.O. Box, Denio, NV 89404, call (775) 941-0270, or e-mail gleln@nevadaopal.com. Write to the Bonanza Opal Mine at 62550 Waugh Rd., Bend, OR 97701, call (541) 383-1700, or e-mail nadine1700@aol.com. For the Royal Peacock Mine, write to Walter Wilson, HCR 0019535, Winnemucca, NV 89445, call (775) 272-3201, or e-mail maestes@frontiernet.net. Some remarkable finds have been made at these; mines through the years by both mine owners and fee diggers.

The Virgin Valley opal, first found in 1908, has been much maligned because it tends to crack. What opal doesn’t? In the early days of mining here, the fellows would dig out a superb piece of opalized wood and leave it on the roof of their tin shack. The sun would abuse it, and if it didn’t shrink and crack, it was a good gem. You can bet that the opal miners in Australia are well aware of their opal’s infrequent but annoying penchant for cracking. We just don’t hear the bad news from there.

As mentioned, Australia is the source of most precious opal on the market today. After all, the deposits from which Austr4alian opal is dug formed from weather invading the rock. Since the water came from surface action like infrequent rains, such deposits are called supergene deposits, and they are!

Many of the important Australian deposits reside in what is called the Great Artesian Basin, a broad, gradually down-slopping area that was once covered with water. Creatures lived in the water, and their remains ended up on the basin floor. Stunning, nearly complete skeletons of plesiosaura and other fantastic creatures have been found replace by lovely precious opal! Not so spectacular, but eagerly sought, are opal replacements of shells, belemnites, and other once-living sea things. Opal seems to have an affinity for once-living things. Think of the wood and even pine cones of Virgin Valley that are now lovely fiery opal. The many opalized animal remains found in Australia could not have formed by chance. I suspect that as the tissue and harder parts of the living thing slowly dissolved, they left spaces where silica-rich waters could seep in and opal could develop, creating in effect pseudomorphs of precious opal after living tissue. Along with the opalized remains, fine small masses and seams of opal are mined here.

Elsewhere in Australia, there are formations of hard, reddish sandstone that, like many sedimentary formations, have long since cracked and split. Silica-laden waters flowed into those cracks and slowly coated the walls of those cracks and joints with tiny spherules. Split open the cracked walls, and you may reveal a rainbow of colors. Usually, this opal is paper thin, but in bright streaks of colors. While often not suited for gem work, this opal on rock is shaped into very attractive desk and shelf ornaments.

There is another opal-producing area in Australia that differs in general form from the Great Artesian Basin. This is the productive area around Coober Pedy, Mintibe, and Andamooka. (Coober Pedy is an aboriginal name that means “man in a hole” a very appropriate name.) These deposits formed when silica-rich water was trapped between layers of clayey sandstone and a montmorillonite clay formation that had altered into argillite, clay hardened to rock by metamorphic action. This argillite forms a rather impervious layer, so when silica-rich water trickles down through the overlying clayey sandstone, it dissolves silica from the sandstone, which is primarily grains of quartz. The water is trapped by the argillite and stagnates, and lies perfectly still and undisturbed for great periods of time, producing idea conditions for opal to form. This allows for the deposition of the silica spherules of the right size. If they are also able to align side by side in rows, they fit tightly together to form the necessary opal structure that will, when later exposed, scatter light into its visible colors.

The earliest Australian opal find was made by a German geologist in 1849, a rather signal year when you consider what happened in California that year. The first recorded discovery of Australian opal, however was in 1972. In 1877, the opal deposits at White Cliffs were found and mining began in earnest there. Some 25 years later, black opal, considered by many the most precious of all the opal forms, was found at Lightning Ridge, also in New South Wales.

Black opal is so called because the groundmass in the opal is a very dark, though not actually black, color. Because it is so dark, it sets of the fiery colors much better than the more common cloudy gray groundmass of other opals. These days, the poorer grades of Australian black opal are sometimes treated to darken the groundmass.

The treatment technique is as old as the Greek treatment for dull gray agate. The specimen, whether agate or opal, is soaked in sugar solution for a long period of time. Then is put into strong sulfuric acid; the Greeks used strong vinegar. The sugar, being composed of carbon, hydrogen and oxygen, will break down, releasing the hydrogen and oxygen as water, while the carbon burns, or carbonizes, the opal, giving the desired dark matrix effect and highlighting the flash of colors in the opal.

There are other treatments; I like the one using motor oil! The treatment involves burying the opal in the ashes of a fire, then pouring a mess of crankcase oil, which is usually pretty black from use, over the opal “grave.” The final act is to set the whole mess on fire and hope the opal survives. If it does, it may come out with a black background!

Of course, we all know now that there are several methods of replicating nature’s fiery opal gems. Simply by figuring out how opal formed in nature, we had the information experimenters needed to replicate the process. Of course, they also figured out how to speed up the process a bit, so we don’t have to wait eons for useful results.

The discovery of the cause of color in opal was made in 1964. Less than 10 years later, scientist Pierre Gilson produced opal in the laboratory. His opals have either white or black backgrounds and fine color, and have been on the market since 1973. The Gilson process is a trade secret of course but probably doesn’t stray too far from the original method, since the goal is to replicate nature’s inordinate skill in creating superb gems.

The first synthesis of opal occurred in Australia when two scientists combined sodium with a resin in the heating process, then let the results settle into a colloidal silica “stack.” The larger spherules settling first, the smallest settling last. From that graduated stack, they were able to select a given layer and extract spherules of one size. They then extracted whatever remaining water there might be, and used a adhesive to create a solid mass the resembled precious opal. By today’s standards, this is a crude method, but it worked and led others to refine the process.

You may have also seen an opal like gem offered at shows called Slocum stone. Named for its developer, Slocum stone is actually a form of glass. Just how John Slocum accomplished his great – and it is quite a feat – of getting glass to look like precious opal is a guarded secret. Still, the results are excellent and can be obtained for use at prices that are generally lower than that of Gilson opal and certainly much less than that of natural opal.

No matter whether you use Slocum stone, Gilson opal, or fine precious opal from any of the worldwide sources in your work, the result will be a thing of beauty, as any gem rough that shoots out flashes of color under an ordinary light has great appeal. After all, beauty is in the eye of the beholder, and what can be more beautiful than beholding a superb precious opal under intense light: And trust me, you won’t be jinxed.

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