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Bicolor Tourmaline

Written by rdjadmin On May 2, 2013.

At last count, gemologists had logged in more than 100 tints for tourmaline, making it the gem world’s king of color. Centuries ago, however, when its broad palette meant constant confusion with other stones, tourmaline was one of the gem world’s masters of disguise. Indeed, the gem’s root-name, is turmali, a Sinhalese term used by Ceylonese dealers and means “mixed gemstones of unproven identities.” Today the original name is still apt because it reflects a gemological diversity so great that the gem is classified as a group rather than a family.

Yet, strangely enough, the gem that comes in more colors than any other begins life as colorless, or nearly so. During formation, its crystal structure hungrily absorbs trace elements from its host solution. Some of these chemical impurities — most notably, manganese for pink and iron for green — act as coloring agents when they replace atoms normally present.

This susceptibility to color change through chemical substitution makes tourmaline what is called an “allochromatic” gem. Of course, tourmaline is far from alone in this trait. In fact, without such color-susceptibility, many revered gems would be missing from jewelry stores. If, for instance, chromium didn’t replace aluminum in beryl, there would be no emerald.

Nevertheless, tourmaline takes to a wider variety of tinting than any other gem. And because it does, explanations of color causes in tourmaline are still largely theoretical and often conflict with one another. When it comes to color, tourmaline seems to play by lots of rules. “In Brazil, the color chemistry for tourmaline changes from pocket to pocket within an occurrence,” gemologist Joel Arem notes, “while in California it remains fairly uniform throughout the whole San Diego gem belt.”

Many tourmalines provide a visual record of changes in their chemistry by displaying two, sometimes three, areas of different color along their crystals. When cut, these multicolor gems become what Arem calls “history books of tourmaline growth.” Most frequently seen with two large zones of adjoining color, the gem is best known as bicolor tourmaline and usually features combinations of pink and green. Both the pinks and greens run wide gamuts from magenta-purple to brownish-pink and aloe to granny-smith apple green, with numerous intermediate shades.

Occasionally, bicolors depart from pink-green pairings and couple spruce-greens with aqua-blues, aqua-blues with cherry-pinks, and grass-greens with pumpkin-oranges, to name a few of the most beautiful combinations. “No doubt, new finds will give us new color combinations,” Arem adds.

Local Colors

At present, bicolor tourmalines are mined principally in Brazil, Zambia, Madagascar, Nigeria, Afghanistan and California.

It would be nice to lay down some hard-and-fast rules for buyers of bicolor tourmaline. But taste with this gem is as wide as its color palette. And that’s how it should be. However, if you insist that stones be eye-clean, this will pretty much restrict you to California, whose Himalaya Mine in tourmaline-rich San Diego County is famous for flawless bicolors. This doesn’t mean San Diego is the one source of flawless stones. Most localities produce some, but nothing proportionate to California. No matter what the origin, flawless bicolor tourmaline will nearly always have lighter hues.

As for color composition, California’s tends to have soft yellow-greens reminiscent of spring grass and pastel purples, often with a touch of brown.

It is hard to generalize about Brazilian color. In addition to green and salmon-pink, some stones show a combination of green with a dusky brownish-purple. Occasionally, stones possess a lovely indicolite-blue that commands an extra premium for rarity.

Zambia is noted for striking grass-green/pumpkin-orange combinations and, once in a while, unique aloe-green/terra cotta pairings. Madagascar’s stones have duskier greens and pinks. Afghanistan’s stones tend toward mint green and pastel pink. Be on the lookout for rare periwinkle-blue/magenta team-ups. Finally, there’s Nigeria, a source known for pairings of the same color, usually light to dark green.

When selecting bicolor tourmaline, expect the ratio of the two colors to be at least 60/40- with 50/50 the ideal. Breaks between colors should be straight up and down with minimal transitions.

Singular Sensation

Bicolored tourmaline has always been a gem for custom jewelry only. The very things about the gem that turn on makers of one-of-a-kind jewelry turn off manufacturers of mass-production jewelry.

Strike one for manufacturers is the unusual shape.While other tourmalines come in cushion, oval and even round shapes, bicolored ones are typically cut only as emerald shapes, many of them extremely thin.

Benitoite

Written by rdjadmin On May 2, 2013.

If mining company owner Rod Dallas hadn’t sent two prospectors up into the southern Mt. Diablo range to look for copper and mercury late in 1906, California might not have its state gem today.

The gem is benitoite, named after the stone’s sole source of facetable material in San Benito County, California, near the Fresno County line. Due in part to the fact that it has only one meaningful occurrence, benitoite became California’s state gem in 1985.

It is highly doubtful, however, that the gem would have been awarded such status just because of its rarity. Beauty and gemological uniqueness (it’s the only gem known in its crystal class) played equally decisive roles. In fact, it can be said California’s lawmakers were acknowledging benitoite’s remarkable and swiftly attained stature among gem collectors. Right after it was identified as a new species, benitoite became a paragon among collector gems.

Shiny Blue Pebbles

Like its brethren state gems, Main tourmaline and Montana sapphire, benitoite was found purely by chance. As so often happens, its discoverers were distracted from the pursuit of metals by the chance glimpse of a mineral-in this case, highly reflective blue pebbles lying on the bank of a small mountain stream at a spot some 4,000 feet above sea level. “It would have been hard not to notice these gems,” explains author Peter Bancroft, who devoted an entire chapter of his 1984 book, “Gem and Crystal Treasures,” to benitoite and who explored for the gem in the 1930s. “Benitoite is found in dry, scrub-infested country where serpentine [a very soft, flaky mineral with a dull green color] is the predominant base rock. Given this terrain, anything bright and shiny would stand out.”

Benitoite is certainly bright and shiny. But even among the ranks of the radiant, few colored stones exhibited brilliance as great as those from the new deposit in California’s Coast Range. So while its denim- to violet-blue colors at first led many jewelers to mistake it for sapphire, some who were intrigued by its unusually high dispersion suspected it was something else altogether.

One skeptic at Shreve & Co., the San Francisco jewelry store that was to be the mystery gem’s earliest marketer, persuaded his superiors to send samples to George Davis Louderback, a geologist at the University of California, Berkeley, for study. In July 1907, he published the first of two papers on the stones, identifying them as a new species that he named “benitoite” for its geographic origin. The name has proven apt since no other deposit-let alone source-of facetable benitoite has ever been found.

For Collectors Only

What is now known simply as the Benitoite Mine is very small. Bill Forrest and Buss Gray, who have owned the mine since 1987, are convinced that if it had been worked all year round once discovered, it would have run dry decades ago. They ought to know, since they’ve worked the mine longer than anyone else-leasing it for 20 years before buying it. For them, a mining season lasts five or six weeks, usually in March and April. “Lack of water becomes so severe a problem by May that we must shut down operations,” Gray says.

Six weeks isn’t time enough to find much benitoite. Maybe that’s why, excepting its brief association with Shreve & Co. between 1907 and 1912, this gem has no other history as a jewelry stone-although Tiffany is said to have tried for an exclusive on the mine’s output once the news broke of benitoite’s existence. This lack of jeweler interest is probably for the best, since benitoite’s one major drawback is a hardness of 6.5, the same as tanzanite, that makes it risky for ring use.

In any case, retailer neglect hasn’t hurt benitoite’s status. To the contrary, immediately after it was announced as a new gem, benitoite ignited an ardor in collector circles which is as strong now as ever. Today, estimates Gray, at least 90 percent of all benitoite is sold to collectors-most, until recently, in Europe. Now, however, a fast-growing audience for the gem is found in America.

From an aesthetic standpoint, it’s easy to see why benitoite has endeared itself to collectors. Fine specimens typically feature flattened, triangular gem crystals, rarely more than one inch wide, that are translucent with white centers and blue edges. These crystals either protrude from or are embedded in granular snowy-white masses of natrolite that serve as matrix. The contrast between these distinctive and well-formed benitoite crystals and their host rock is dramatic and impressive.

Further aesthetic delights are revealed once these crystals are extracted (using hydrochloric acid) from the natrolite for faceting. In color, faceted stones most often resemble sapphire and tanzanite, with darker stones more like the later, lighter ones the former. On rare occasion, cut stones are either pink or colorless. Unless stones are overly dark, something rare for this breed, benitoite looks stunningly brilliant-the result of dispersion only a tad less than that of diamond.

Small is All

For those who like their gems to possess girth, benitoite may be a disappointment. Since crystals are usually highly included, only small areas can be cut that are eye-clean. What’s more, the flatness of most benitoite crystals restricts the cutter’s maneuverability and chances for good recovery. Last, best color is attained by cutting along a crystal direction that requires considerable rough to be sacrificed.

Consequently, stones most often finish to sizes between 10 points and slightly over 1 carat-regardless of the size of the crystal from which they originate.

“On average, we cut five stones a year that weigh 2 carats or more,” says Gray. “We’re lucky if we find one crystal every five years that yields a stone over 5 carats.”

To date, the largest faceted benitoite known weighs 7.80 carats and is on display at the Smithsonian Institution in Washington D.C.

Australian White Opal

Written by rdjadmin On May 2, 2013.

Opal was highly valued in Ancient Rome. But the gems they treasured wouldn’t rate a second glance today. The opal of the ancient world was a cloudy white gem with a few subtle pin pricks of play of color. This opal was mined in what is now the Czech Republic, near Czerwenitza.

Since the discovery of Australia’s incredible opal deposits in the nineteenth century, the standard for beauty in opal has been redefined. Australia’s opals have more fire, more transparency, and more abundance than the ancients dreamed possible. Thanks to the richness of Australia’s deposits, white opal has become a staple in affordable fine jewelry. The average consumer can own an opal that would put the opals of emperors to shame.

Pretty For Pennies

Mass merchandisers have found opal an ideal choice for affordable jewelry, especially earrings and pendants. For extremely little money, white opal provides a beautiful gem, that is unique, thanks to its individual pattern of play of color. According to a New York dealer, the most popular types of white opal in the mass market are: 1) milky white with pale multi-colored pinfire, or 2) blue green with broad flashes of color.

Because a large amount of low-end opal is used in earrings, pinfire is more popular than broad flash: it’s easier to match pairs.

Up The Ladder

Consumers who pride themselves on wearing quality will probably not be happy with largely opaque, pale, no-color-play opal- no matter how cheap. This opal hardly lives up to its name, derived, in part, from the Greek word opallios, which means “to see a color change.” But to get stones with an interesting array of color, plus some translucency, expect to pay a lot more than is asked for run-of-the-mill anemic opal. Stones that boast strong foreground reds, greens and blues are definitely not available at trinket prices.

The next step up is what dealers call “crystal opal.” This term refers to the look and not the structure of stones. These opals sport, in one dealer’s words, “a translucent glass-like appearance with full colors seemingly suspended in a transparent base.”

But “crystal opal” is only one of the varieties of better opal. The finest usually show no identifiable background color, just a continuous, unbroken array of vivid color patches or patterns. These stones will cost about the same as better aquamarine or imperial topaz.

Although commercial-grade opal is largely what jewelry manufacturers use these days, there seem to be strong stirrings of collector interest in better opal.

Improving the Odds

Opal has a weakness: some stones eventually crack because of structural weaknesses made vulnerable by heat and dehydration. But the problem can be minimized by dealer quality control.

First, the knowledgeable dealer knows which locations in Australia produce the most stable opal (Andamooka is famous for such stones), and shops accordingly. As an extra safeguard, most dealers hold all polished stones for several weeks and return any that crack.

Second, because those opals that crack tend to do so sooner rather than later, the responsible dealer will refrain from selling newly purchased or polished stones for a certain period of time to let nature weed out any losers. Dealers we interviewed call this procedure “curing” and subject both rough and cut stones to it.
Unfortunately, the dealer’s ounce of prevention can be undone by abuse in display cases.

Since those opals with a high water content are the most prone to cracking (also called “crazing”), prolonged exposure to bright lights in a closed, unventilated showcase is an invitation to trouble. Thankfully, cracks are often skin deep and can be buffed out on a polisher’s wheel.

Australian Sapphire

Written by rdjadmin On May 2, 2013.

Connoisseurs may glorify rare Kashmir sapphire with its soft velvety blue. Collectors may extol the hard-to-find Burma breed with its crisp royal color. And specialists in swank gems may laud the Sri Lankan variety for its cool stately hues.

But few sing the praises of Australian sapphire, even though it was about the only variety known to the common man for nearly 30 years until Thai sapphire, and later African, supplanted it as a market mainstay. Now it is unlikely anyone ever will.

For Australia’s claim to fame was in the realm of quantity, not quality. Indeed, it was to bulk what Kashmir was to beauty. Since the early 1960s, it had enjoyed unrivaled leadership as a sapphire producer. Then between 1985 and 1991, it went from supplying 70 percent of the world’s sapphire to less than 20 percent. That killed Australia’s reputation as the key source of bread-and-butter stones. Even when it was, dealers disliked its dark-toned, slightly greenish-colored goods.

Strangely, the consistent inkiness of color disparaged by connoisseurs made this sapphire indispensable to mass manufacturers and merchandisers. As late as 1989, dealers assumed that any mass-produced piece of sapphire jewelry used Australian material because it was the only reliable source of matchable blue tones.

Because America was the world’s chief manufacturing center for mass-production jewelry, the vast majority of Australia’s blue sapphires wound up here. But as chain stores became a bigger factor in Europe and as new markets for low-end jewelry emerged in the Pacific Rim area, exploding demand for inexpensive sapphire suited to mass production drove up prices for all but the lowest-grade Australian goods at least 50 percent in just two years.

Spiraling prices prompted exploration for sapphire in Thailand, long a producer of the gem. And since the Thais bought and cut 90 percent of Australia’s production, success with mining at home was sure to hurt sales in Australia. Matters weren’t helped by the fact that Australian sapphire resisted oven touch ups.

Heat Treat

Ever since meaningful sapphire production began in Australia around 1890, bringing these gems to market has always been handled by people as strongly versed in gem enhancement as they are in cutting.

Dealers from Idar-Oberstein, Germany’s centuries-old cutting center, were the first market-makers for Australian sapphire. They bought the lion’s share of rough found at the Anakie alluvial fields in central Queensland (Australia’s most significant deposit until 1959 when large-scale mechanized mining began at the New England fields of northern New South Wales), processed it back home, then sold the finished goods to Czarist Russia. World War I and the Russian Revolution of 1917 ended Australian sapphire’s initial period of popularity.

Part of that processing may very well have included some form of heat treatment. How else could the Germans have succeeded with Australian sapphire given the gem trade’s then somewhat low esteem for this material. In 1908, gemologist Max Bauer wrote rather unflatteringly that “Australian sapphires, as a rule, are too dark to be of much value as gems.” Very likely the Germans were quietly broadening the new sapphire’s appeal the way they had broadened the appeal of Brazilian aquamarine-namely, by heat-treating stones to improve color.

Certainly, oven alchemy is the key to Australian sapphire’s second and far greater wave of popularity in our time. Using heat to lighten up the dark color, dealers from Thailand, the main buyers of Australian sapphire from 1960 on, were able to create a market for what otherwise would have been low quality rough. For more than 20 years, about 10 firms were buying an estimated 90 percent of the output from Australia’s fields.

Heat dissolves the heavy silk that is a major internal characteristic of Australian sapphire. Unless this silk is removed, Australian sapphires tend to be plagued by pronounced blue/greenish-blue dichroism (transmission of two different colors when a gem is viewed in different directions) that imparts an objectionable greenish cast to stones. From the look of recent purportedly Australian sapphires we’ve seen, the heat treaters of Bangkok have become quite expert at minimizing annoying green.

As the market for Australian blue sapphire has declined over the last decade, miners have started to look at yellow and green sapphires from these deposits. It’s ironic: sapphires that were originally dubbed unmarketable for the yellow and green colors, as overtones to their blue, may eventually become celebrated for those colors themselves.

Australian Black Opal

Written by rdjadmin On May 2, 2013.

O, the power of the pen. With just one strange plot twist in his 1829 bestseller, “Anne of Geierstein,” Sir Walter Scott destroyed the European opal market for nearly 50 years. And he did it just by having a character falsely accused of being a demoness die shortly after a chance drop of holy water fell on her opal and quenched its mysterious, fiery color. Convinced Scott was warning them that wearing opal could bring bad luck, suggestible readers stopped buying the gem. Within a matter of months, the opal market had crashed and prices were down more than 50 percent.

It took a remarkable find of black opal at Lightning Ridge in New South Wales, Australia, in 1877, to revive the market for this gem. Australia’s spectacular new opals took the world by storm. Indeed, there have been almost no opals found in the last 50 years to match what came from Lightning Ridge in its late nineteenth and early twentieth century heyday. No wonder dealers sometimes liken Lightning Ridge’s impact and significance to that of Kashmir for sapphire, a Himalayan source discovered around the same time, that set the standard for sapphire in the same way Lightning Ridge did for opal.

Lightning Strikes Once

Black opal is far rarer than white opal. In fact, there are only two active black opal localities in the world, both in Australia. Of the two, the older, Lightning Ridge, is said to be the only one to produce true black opal. The other, Mintabe, is known more for gray-to-grayish-black material. Some purists even protest calling Mintabe stones black opal. They do so on both geological and aesthetic grounds.

Traditionally, black opal was considered a variety of opal found in rock formations called nodules (or “nobbies” in the trade) as opposed to white opal, which is found in seams. But when Mintabe, whose opal is also found in seams, was discovered, this geology-based distinction began to blur- at least among dealers.

One can see why opal dealers insist that Mintabe’s dark- gray material be accorded full standing as black opal. If they had to restrict their definition to goods from fast-depleting Lightning Ridge, the future of this gem would be in grave doubt. For even when production from Lightning Ridge and Mintabe are pooled together, there is still far less black than white opal. Given its relative scarcity, black opal has never lent itself to volume jewelry use the way white opal has. As a result, black opal remains primarily a special order stone in this country. But the few who buy it here generally look for the same things the Japanese do: base color, hue mixture and color pattern.

Basics of Opal Beauty

When dealers evaluate black opal, they start with its base color. The darker the base- or what dealers call “potch”- of an opal, the more pronounced and vivid its color. That’s because an opal’s beauty is the product of the contrast between its color play and its background.

As for ideal color in black opal, most experts look for the predominance of red and then orange. “Stones that appear all or almost all red are the most coveted,” one dealer explains. “Increasing amounts of blue and green will reduce their value.”

But specific color isn’t all that connoisseurs want in a fine black opal. The pattern that they take is equally important. For years, the most prized pattern was what dealers call the “harlequin”: well-defined squares, rectangles, triangles or diamonds of color in both symmetrical and asymmetrical designs.

Today, unfortunately, the harlequin pattern is encountered only in older pieces. No new stones with this pattern have been mined in years. Failing to find stones with big blocks or even chunks of color, connoisseurs must content themselves instead with swirls. Often you’ll hear them describe the best patterns that can be hoped for in black opal today as stones that exhibit broad well-defined flashes of color (called “rolling flash” in the trade) that change kaleidoscopically as stones are turned in the hand. Yet even these patterns are far from plentiful.

Realistically, the top black opals that consumers are likely to be offered nowadays feature color patterns comprised, at their best, of lively pointillistic dots. As these dots get smaller and less vivid, they come to resemble what the trade calls “pin fire.” Such black opals are the most commonly found and least expensive.


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