Gemology 101: Introduction to Gemology
Article: Handling Colored Stones at the Bench Part 2
I'm hoping this series of articles will save some of you time, money, and embarrassment by discussing some of the common bench procedures that can easily lead to damage of colored stones - and how to avoid it. Some of it will be familiar, some of it new, and some of it controversial. At the end of the series I'll be offering a concise guide to help with bench related questions about colored stones.
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Andalusite
Not commonly seen in recent years, andalusite can be a real beauty. When properly cut, the stone displays two colors at once (pleochroism); in the case of a long oval, light olive green in the middle and pink to brownish-red on each end. Definitely an attention-getter!
Named after the locality where it was first discovered, Andalusia (Spain), Brazil is the primary source today, with some material coming from Sri Lanka and East Africa. Most cut gems are in the 1-5 carat range, with nice stones over 10 carats quite rare.
The Mohs hardness has been reported at anywhere from 6.5 to 7.5, So it is quite hard, but andalusite is somewhat brittle, And care should be used in setting. Bezels are not advised.
One to three carat stones wholesale from $50-75 per carat in Fine quality, up to $90 per carat in extra fine and well cut. Five to ten carat stones, when available, range up to $27 5 per carat.
Aside from the slight brittleness, andalusite presents no handling difficulties, but direct heat or sudden heat changes should be avoided. Pickle is not a problem, nor is ultrasonic cleaning.
Apatite
Until the appearance of neon blue-green apatite about ten years ago, the:re was no interest in apatite as a jewelry stone. The large yellow crystals from Cerro Mercado, Mexico were always in demand by cutters like me, but that material is pretty much gone. While apatite cuts a beautiful and bright stone, the Mohs hardness does not exceed 5 in gem quality material, and it is quite brittle. All in all, this is not a good candidate for a ring, but the popularity of the newer neon colored material is not to be ignored. In addition to its brittleness, apatite is very heat sensitive. Torch use is out and the ultrasonic is risky at best. Extreme caution is advised when setting. It's almost in the "look but don't [ouch" category.
Besides the currently popular neon blue-green, apatite can be found in a nice medium yellow, light pink, greenish-yellow, even purple, but the material is never common. Extra fine faceted apatite in the neon colors wholesales in the $150 to $175 per carat range in five to ten carat sizes and is mostly sold to collectors. When well cut it is a visual knockout! Under 3 carats is usually available for under $85 per carat.
Aquamarine
Aquamarine is the blue to greenish-blue variety of the mineral beryl. It can range from a very pale blue to fairly intense blue or greenish blue. Aquamarine is almost always heat treated, sometimes at the mine site, but usually after cutting, to drive off the greenish or yellowish component, leaving a more pure, and popular, blue hue. This has been done for so long now that younger buyers often do not associate the natural greenish-blue color with aquamarine ! However, the unheated material is always sought after by cutters as it is highly regarded by gem connoisseurs and fine jewelers.
Brazil and Pakistan are large producers of aquamarine, and it is found in many locations around the world, including North Carolina and some excellent specimens come from Mt. Antero, Colorado. The very finest material, a rather intense pure blue similar to "London" blue topaz. known as o'Santa Maria" (named after it's Brazilian location), has long disappeared from the marketplace, and the finest source of high grade aquamarine today is Mozambique. This material is known as "Santa Maria Afrique", and the very limited supply has doubled in price in the past few months.
Three to ten carat top Brazilian goods range from $100 to $600 per carat, depending on intensity of color, while fine cut Santa Maria Afrique ranges easily to $1000 per carat. No price differential is seen between heat-treated and natural gems. Aquamarine, along with tourmaline and spinel, has become one of the fastest appreciating gems in terms of cost.
The hardness of aquamarine is usually reported as 7 .5 to 8, but some gems have been tested and shown to be less than 7 , so some caution is advised. Aquamarine is not especially brittle, but heat should be avoided. The stone itself is not particularly heat sensitive, but it may contain invisible liquid-filled inclusions which, when heated, turn to gas as they expand, and can shatter a stone. If heated much hotter than 450 degrees Centigrade, aquamarine is likely to turn chalky white or colorless. The ultrasonic is safe if you use caution not to plunge the stone from room temperature to boiling. Pickle presents no problems, but the stone should be allowed to cool first.
The Russians have been successful in creating synthetic aquamarine. As the price for darker natural aquamarine continues to skyrocket, we can expect to see more of this material in the marketplace, although the price of the synthetic keeps it in the "luxury" synthetic class. I see prices from $30 per carat for poorly cut, big-bellied material with windows to $80 per carat for properly cut, brilliant goods in 3-7 carat sizes. The synthetic is generally very clean and therefore heat resistant to 700 degrees Centigrade or so, but you'll "purify" the color at around 500 degrees. So, avoid direct torch, but otherwise treat it the same as natural.
Beryl
The color varieties of the mineral beryl provide us with some of our most beautiful gemstones. These include aquamarine (blue to greenish-blue), emerald (moderate to intense green to bluish-green), Heliodor or golden beryl, Morganite (pink to pale purple-red), Hiddenite (light green), Goshenite (colorless), and the extremely rare Bixbite or red beryl. Most members of the beryl family are fairly durable and offer no particular problems to the bench jeweler, with the exception of emerald or any of the other varieties which may be heavily included. Each of the varieties is discussed separately, but high or sudden heating or cooling is always to be avoided with all the varieties of beryl.
Chrome Diopside
Until recently, this intense green stone was rarely seen in jewelry. Recent discoveries in Russia and Madagascar have increased the availability and the TV shopping networks are doing their part to make it popular, so we can expect to see these stones at the bench with increasing frequency. All diopside ranges in hardness from about 5.5 to 7, and it is not exceptionally brittle.
The gem varieties are remarkably resistant to heat, but sudden quenching is a no-no. The ultrasonic and the pickle present no problems.
Most of the chrome diopside you are likely to encounter will be under two carats, because stones much larger than that start to app ear black rather than green. You may also encounter diopside in another form, as a cabochon. This material is dark golden brown to black and displays a very nice four-rayed star. It is sometimes confused with black star sapphire, but the sapphire displays six rays, not four. The star variety should NOT be heated or quenched suddenly.
Black star diopside trades at a low value, just a few dollars per carat, while chrome (green) diopside in faceted form trades for $30-75 per carat in three carat sizes, much less in smaller sizes.
Chrysoberyl
The mineral chrysoberyl provides us fantastic gemstones in three distinct forms. The first, alexandrite, the color-change variety has been discussed above. The second is cat's-eye chrysoberyl, the rarest and by far the most expensive of the gems capable of producing the cat's-eye phenomenon. The cat's eye phenomenon, called chatoyancy by gemologists, is caused by the internal structure of the stone. In this case, parallel flbers, needles, or elongated crystals of the mineral itself reflect light in such a way that sheen canbe seen in reflected light on a flat polished surface. When the surface is domed or cut "en cabochon" the eye is seen; the finer the needles, the
better the eye. In the best chrysoberyl the eye seems to glow and as the stone is turned the eye will seem to open and close or "wink".
Varying in color from orange-yellow through yellowish green to grey-green, top cat's eye chrysoberyls in three to five carat sizes sell for $ 1500 to $3500 per carat wholesale. Some also show a color change, in which case they are cat's-eye alexandrite, and the prices go through the roof.
The third variety of gem chrysoberyl is simply transparent yellow-green, usually seen in faceted form. It is generally a medium light yellow to yellowish green. Properly cut it is very brilliant and often free of inclusions. It shares the high 8.5 hardness of its siblings and is very tough as well. Avoid sudden heat or quenching, as usual; otherwise chrysoberyl offers no challenges.
Citrine
Citrine is the yellow variety of quartz. The color actually ranges from a very pale yellow through the yellow-orange to orange color of the best Rio Grande citrine to the darker reddish-oranges of Madeira and reddish browns of what is called oxblood citrine by many.
Many jewelers do not hold citrine in exceptionally high regard, but the darker yellow and bright orange material is not easy to find and seldom seen. When cut to proper angles and polished properly, it is stunning and creates beautiful and affordable jewelry.
While citrine does occur naturally on occasion, most of it is the result of purposely heating amethyst. Many bench jewelers have had the unpleasant experience of getting the torch too close to an amethyst or citrine winding up with colorless quartz and a very unhappy customer. I know it worked for me! once done, it's done, so watch the heat around citrine and amethyst. Barring sudden temperature changes, neither the ultrasonic or pickle should present a problem.
Yellow citrine ranges from a very few dollars a carat for native cut light stones to about $70 per carat as it approaches orange and is custom cut and polished. Orange to reddish-orange material ranges from about $20 per carat to $80 carat for custom cut stones, although this material is increasingly difficult to find, because the amethyst required to produce it brings high prices in the custom cut market.
Both the Russians and Japanese produce large amounts of synthetic citrine for the gemstone market. Actually, they produce amethyst, and heat treat the material just the same as if it were natural. Because the material IS amethyst, it responds by turning some color of yellow to yellow-orange to reddish-brown. Just as with amethyst, lab grown citrine is available for anywhere from a few dollars per carat for yellow, native cut goods to upwards of $70-100 per carat for precision or concave cut stones, where one is essentially paying the labor charge of a highly skilled artisan.
The synthetic material is fairly heat resistant, but can be changed to colorless material by overheating, and it will crack if suddenly quenched. The ultrasonic can be safely used, and pickle has no effect on the quartz family.
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