Terms used to describe optical effects in faceted and non-faceted gems
Physical characteristics of colored stones are often described in terms of the way light travels through them, their unique visual effects, and the way they are cut. Here are a few terms you need to know:
Transparent. Light travels through the stone easily, with minimal distortion, enabling one to see through it easily.
Translucent. The stone transmits light but diffuses it, crating an effect like frosted glass. If you tried to read through such a stone, the print will be darkened and obscured.
Opaque. Transmits no light. You can not see through it even at a tin edge.
Special optical effects
Adularescence. A billowy, movable, colored cloud effect seen in some gemstones, such as moonstones; and internal, movable sheen.
Asterism. Used to describe the display of a star effect (four or six rayed) seen when a stone is cut in a non-faceted style. Star ruby, garnet, and sapphire.
Chatoyancy. The effect produced in some gemstones (when cut in a cabochon style) of a thin, bright line across the stone that usually moves as the stone is moved from side to side; sometimes called a cat's eye effect.
Iridescence. A rainbow color effect produced by a thin film of air or liquid within the stone. Most iridescence seen in gemstones is the result of a crack breaking their surface. This detracts from the value, even if it looks pretty.
Luster. Usually refers to the surface of a gemstone and the degree to which it reflects light. Seen as the shine on the stone. Diamond, for example, has much greater luster than amethyst. Pearls are also evaluated for their luster, but pearls have a softer, silkier looking reflection than other gems. The luster in pearls is often called "orient."
Play of color. Used frequently to describe the fire seen in opal.
Cut
Colored gems can be faceted or cut in the cabochon, or non-faceted, style. Generally speaking, the preference in the United States until recently was for faceted gems, so the finest material was usually faceted. However, this was not always the case in other eras and other countries; in Roman times, for example, it was considered vulgar to wear a faceted stone. Preference also varies with different cultures and religion and the world's finest gems are cut in both styles. Don't draw any conclusions about quality solely on style of cut.
Cabochon. A facet-less style of cutting that produces smooth rather than faceted surfaces. These cuts can be almost any shape. some are round with high domes; others look like square shape domes (the popular "sugar-loaf" cabochon); others are "buff-topped," showing a some what flattened top.
Many people around the world prefer the quieter, often more mysterious personality of the cabochon. Some connoisseurs believe cabochons produce a richer color. Whatever the case, today we are seeing much more interest and appreciation for cabochons around the world, and more beautiful cabochons than have been seen in the market in many years.
Faceted. A style of cutting that consists of giving to the stone many small faces at varying angles to one another, as in various diamond cuts. The placement, angle, and shape of the faces, or facets, is carefully planned and executed to show the stone's inherent beauty; fire, color, brilliance, to the fullest advantage. Today, there are many faceted styles, including "fantasy" cut which combine rounded surfaces with sculpted backs.
The importance of cut
Cutting and proportioning in colored stones are important for two main reasons:
They affect the depth of color seen in the stone.
They affect the liveliness projected by the stone.
Color and cutting are the most important criteria in determining the beauty of colored stone, after which carat weight must be factored in; the higher carat weight will usually increase the price per carat, generally in a nonlinear proportion. If a colored gemstone was a good quality material to begin with, a good cut will enhance its natural beauty to the fullest and allow it to exhibit its finest color and liveliness. If the same material is cut poorly, its natural beauty will be lessened, causing it to look dark, too light, or even "dead."
Therefore, when you examine a colored stone that looks lively to your eye and has good color; not too dark and too pale, you can assume the cut is reasonably good. If the gemstone's color is poor, or if it lacks liveliness, you must examine i for proper cut. If it has been cut properly, you can assume the basic material was poor. If the cut is poor, however, the material may be very good and can perhaps be re-cut again into a beautiful gem. In his case you may wan to confer with a knowledgeable cutter to see if it is worthwhile to re-cut, considering cutting costs and loss in weight.
Evaluating the cut of a colored gem
When examining the gemstone for proper cut, a few considerations should guide you:
Is the shade pleasing, and does the stone have life and brilliant?
If the answer is yes to both questions, then the basic material is probably good, and you must make a decision based on your personal preferences and budget.
Is the color too light or too dark?
If so, and the cut looks good, the basic uncut material was probably too light or too dark to begin with. Consider purchase only if you find the stone pleasing, and only if the price is right, i.e., significantly lower than gemstones of better color.
Is the gemstone's brilliance even, or are there dead spots or flat areas?
Often the brilliance in colored gemstone is not uniform. If the color is exceptional, subdued brilliance may not have a dramatic effect on its allure, desirability, or value. However, the less fine the color, the more important brilliance becomes.
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