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.
While not strictly a gem material (it is an organic substance not a mineral), its use in jewelry throughout history was common. In many places red coral is now a protected species, and it is becoming less commonly seen, but there remain many fine pieces, some very highly prized and expensive, of red or pink “angelskin” coral cabochons, intaglios and cameos.
Coral is calcium carbonate, which is actually a form of the fragile mineral calcite. It ranges in color from white to pale pink, salmon pink, pale to deep rose- red and dark red. Rarely, black coral is found and is highly prized in some parts of the world. Coral accepts dyes readily and is dyed many colors. These dyes range from persistent aniline dyes to ordinary vegetable dye, and may not be resistant to cleaning, so test any cleaning method on an unexposed portion of the piece in question. Coral may be banded or zoned, and always shows a cellular structure, although you may need a loupe to see it.
All coral is rather soft and easily scratched. Acids of any sort must be avoided, especially pickle. Coral is very heat sensitive, and some ultrasonic cleaning solutions are not safe.
As with most fragile lapidary materials, the best way to clean coral is with a mild liquid hand soap and warm water.
Pink, red and salmon colors in smaller sizes are sold on a per stone basis, with red being the most expensive at about $50-60 for a 12 x 10 mm cabochon. Pink coral will usually be about a quarter of that cost. As the size increases, so do the cost differentials. Currently a line pink cab measuring 18 x 13 costs about $60 wholesale, but a red of the same size could cost as much as $50 per carat! Be careful!
Coral simulants include glass, plastic and proprietary material made especially to imitate the various colors of coral. All should be treated carefully, as if they were coral.
Corundum is the mineral name for aluminum oxide. In gem quality it is called sapphire, and occurs in almost every color of the rainbow. A red sapphire is, by definition ruby, but all other colors besides red or intense pinkish-red are called sapphire. Sapphire and ruby are among the hardest and most durable of all gem stones, and, in the natural world, only diamond is harder.
Blue sapphire is the most popular gemstone in the United States and, in its finer qualities can be quite costly, especially in sizes over three carats, Aside from fine ruby, the most expensive color is padparadscha (lotus blossom), an intense orangey-pink.
Almost all sapphire and ruby is treated in one way or another. The most common and accepted form is heat treatment. Depending on the chemistry of the individual stone, heat treatment of various types can be used to lighten overly saturated stones, intensify pale ones, and diminish the “silk” (usually fine crystals of rutile) so common in corundum, thereby improving the clarity. The amount of heat involved is higher than what will be encountered in the shop, ordinarily, so the cautious use of a torch and some form of heat protection is usually all that is needed when working with sapphire that has only been heat treated.
Unfortunately, many other forms of treatment are becoming more prevalent, and these can cause trouble. Most rubies being mined are not only heat treated, but is heated with a borax compound which forms a glass at high temperature that fills the numerous cracks and fissures so common in the material available today. This is quite an ordinary glass, often nearly invisible, and is easily melted or partially melted by the torch. When that happens, a distinct haze or whitish frosting appears on the surface, and must be polished out.
Sometimes, and perhaps more commonly, a lead or bismuth compound is used to fill the surface reaching fractures, making them invisible. This is similar to fracture filling in a diamond, and it takes little heat to damage such a stone. I strongly suggest that ruby be removed from the mounting before work involving heat commences.
Sapphire left for many hours in a warm pickle solution can develop a very thin surface oxidation that looks like oil on water. Sometimes this can be removed by rubbing the loose stone briskly between your hands with a tablespoon of ordinary table salt, but sometimes this surface needs to be re-polished by a lapidary. Sapphires worn for many years also can develop this surface, especially blue stones.
A more recent type of corundum treatment involves the use of beryllium, and, perhaps, lithium.
When heated to very elevated temperatures near the melting point for ten hours or more, many of the natural inclusions in sapphire begin to dissolve in the presence of beryllium. In some instances, the beryllium penetrates completely to the center of the stone and can cause startling color changes. Pastel, off-color corundum, or overly dark material formerly of little value, can now be treated with beryllium to create stunning orange, pink-orange, yellow, lilac, and even blue colors (see photo).
Sometimes this treatment is possible to detect with a loupe or microscope, but sometimes it challenges the finest labs. Occasionally when the inclusions have been only partially melted, a blue “halo” can be seen surrounding the partially melted crystals, especially in ruby (see photo).
What effect these treatments will have on corundum in the future is open to speculation, but for our purposes, it is moot. The beryllium treatment is permanent, and ordinary cautious heat from a torch will present no problems, nor will use of an ultrasonic cleaner. Remember, however that when using an ultrasonic with hot cleaning solution or a steamer, it is always wise to bring the temperature of the stone up gradually to avoid the risk of thermal shock and cracking.
Laboratory grown (synthetic) sapphire or ruby is readily available in a wide range of colors. Because of its purity, synthetic sapphire is somewhat harder then natural. Some of this material, particularly ruby, (see photo) is grown by the Czochralski pulling method for laser production and the resulting crystal is nearly perfect at the atomic level. This material is a favorite of many cutters, including myself. It is nearly indestructible and can easily handle torch heat if not excessive or very prolonged.
The “star” in star sapphire (asterism) is caused by microscopic needles of rutile intersecting at sixty degrees. When a translucent or transparent sapphire containing these inclusions is cut “en cabochon” and highly polished, light reflects from the inclusions in such a way that a six-legged “star” is visible.
Star sapphire occurs in many colors, including black. These stones, like all corundum, are quite tough and heat resistant, but common sense should be used to avoid excessive heat. Once in a while you may encounter a black stone with a four-rayed star instead of six. This is star diopside, often sold to tourists in Asia as sapphire. It is inexpensive, although rarely seen anymore, and quite beautiful in its own right. It is not nearly so durable as sapphire, and is prone to damage from moderate heat. If you see four rays, no heat, please!
Fine sapphire is found in many parts of the world including Sri Lanka, Thailand, Burma, Vietnam, the United States, Australia, Tanzania, Kenya, Madagascar, Cambodia, Pakistan and Afghanistan.
Cubic zirconia is sometimes confused with zircon, but zircon is a natural gem material, while cubic zirconia is grown in the laboratory. Optically, it is very close to diamond in its refractive index and similar in dispersion. A properly cut and polished colorless cubic zirconia is difficult to separate from diamond by eye when mounted. However, most cubic zirconia seen in the trade is mass produced on huge cutting machines and exhibits slightly rounded facet edges and poor meets, and often is not polished extremely well.
Cubic zirconia is about 8.5 on the Mohs hardness scale, but it is somewhat brittle. The hardness and brittleness vary somewhat between the colorless material and the various colored and color-change cubic zirconia sometimes available. The photos show Peach & Blue Cubic Zirconia with their rough.
Generally, cubic zirconia can take quite a bit of heat, but it’s better to avoid sudden heating or cooling. It is impervious to most acids and pickle will not harm it, nor will the ultrasonic cleaner.