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Excerpts from the book:
Secrets of the gem trade (the connoisseur's
guide to precious gemstones)
The second-finest shade of green emerald is to be found in stones from the original Chivor mine. This shade tends to have more blue than the Muzo shade. To some neophytes the Chivor blue-green stones appear, at first sight, to have more warmth and fire. The Muzo stones often appear to be over-dark or to have a hint of yellow in them.
Benjamin Zucker, 1984
Though emerald is undoubtedly precious, it is certainly not among the new precious stones. Before the discovery of emerald in Colombia in the sixteenth century, the green variety of corundum was called oriental emerald. The only source of true emerald known to early Mediterranean cultures was the fabled Cleopatra’s mines in the Sinai Desert.
Two mines, both discovered — or rather stolen from their Native American owners by Spanish conquistadors — in the sixteenth century, have set the standard for evaluating fine emerald. Separated by less than one hundred miles, these two mines, Muzo and Chivor, produce emerald that is superior in all respects to the pale, highly included stones from the mines of ancient Egypt.
Chivor was in production first, sometime before 1555. Muzo, a source of larger and even finer crystals, was located five years later. The chief difference between emeralds found at Muzo and Chivor lies in the secondary hue. Geologically, the Chivor deposit is older; these stones tend to have a slightly bluish secondary hue, while those from Muzo tend slightly toward the yellow.
Although some connoisseurs maintain that stones mined in the sixteenth and seventeenth centuries, called “old mine” emerald, were the finest of the fine, Colombia is still a major source of fine emerald.
About three years ago a new source was discovered in an area near Muzo called La Pita. Stones from this source are almost impossible to separate from Muzo stones. Other recent discoveries in Pakistan, Afghanistan, the Brazilian states of Bahia and Goiás and lately in Hiddenite, North Carolina116 now produce stones that rival some of the best that Colombia has to offer.
Emerald is usually either a bit yellowish or a bit bluish; in practice, it may have a bit of both as secondary hues. A slightly bluish secondary hue (ten to fifteen percent) adds a richness and warmth to the overall appearance of the gem. For this reason it is a more desirable secondary hue than yellow. As always, the gem should be examined in all light sources. Incandescent light will bring out the blue. This is the primary reason why some connoisseurs prefer an emerald that is slightly yellowish in daylight. They feel that a bit of yellow balances against the tendency toward blue under the light bulb. If the stone shows more than a fifteen percent blue secondary hue in incandescent lighting it is overblue. African emerald from Zambia often suffers this fault.
Saturation and tone
Clarity and crystal
Most aficionados prefer emeralds cut in the traditional step or emerald cut. This cut has seventeen long, narrow, steplike facets. A majority of emeralds are cut in this style, both because it accentuates the warm satiny hue of the gem and, by happy coincidence, it is usually the most efficient use of the rough material.
Current industry opinion is that oiling and, to a lesser extent, the use of unhardened polymers is acceptable, though dyeing is not. However, in practice, many dealers turn a blind eye to both types of enhancement. Given the high prices of fine emerald, collectors are advised to insist on full written disclosure backed up by an expert laboratory analysis before finalizing the purchase of an emerald.
Recently many labs around the world have adopted a uniform seven-step classification to describe emeralds that have been treated with colorless oils or polymers. The classifications are as follows: none, no significant, faint, faint to moderate, moderate to strong, strong to prominent, and prominent. Stones that fall into the first four classifications, none to faint to moderate, are rare and worthy of consideration. Regardless of appearance, stones that fall into the last two classifications, strong and strong to prominent, are best avoided. The aficionado should bear in mind that these are grade levels of treatment, not clarity. That is to say, an emerald graded none is not necessarily a flawless stone, it is simply a stone that has not been oiled or treated with polymers. An untreated emerald can still look like a piece of a broken Coke bottle. A stone may appear flawless to the eye and still be found to have a’ prominent level of treatment. This means that the treatment has effectively covered up many sins.
The rarity factor
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