Terms used to describe optical effects in faceted and unfaceted opal stone
The physical properties of colored stones are often described in terms of the way light passes through them, their unique visual effects, and the way they are cut.
Transparent. Light passes through the stone easily, with minimal distortion, making it easy to see through.
Translucent. The stone transmits light but diffuses it, creating an effect like frosted glass. If you were to try to read through such a stone, the print would be darkened and obscured.
Opaque. It does not transmit any light. You can't even see through it to the sheet metal.
Special optical effects
Adularescence. A puffy, moving, colored cloud effect seen in some opal rings such as moonstones; and an inner, moving luster.
Asterism. Used to describe the display of a star (four or six rays) effect seen when the stone is cut in a non-faceted style. Star ruby, garnet and sapphire.
Chatoyancy. The effect is created in some gemstones (when cut in the cabochon style) of a thin, light line across the stone that usually moves as the stone is moved from side to side; sometimes called the cat's eye effect.
Iridescence. An iridescent color effect created by a thin film of air or liquid inside the stone. Most of the iridescence seen in gemstones is the result of a crack breaking their surface. This detracts from the value, even if it looks nice.
Gloss. It usually refers to the surface of the gemstone and the degree to which it reflects light. Seen as a sheen on a stone. For example, a diamond has much more luster than an amethyst. Pearls are also valued for their luster, but pearls have a softer, silkier-looking reflection than other gemstones. The luster of pearls is often called "orient".
Play of colors. Often used to describe the fire seen in opal.
Shear
Colored gemstones can be faceted or cut in a cabochon or unfaceted style. Generally speaking, faceted gemstones were preferred in the United States until recently, so the finest material was usually faceted. In other times and other countries, however, this was not always the case; in Roman times, for example, it was considered vulgar to wear a faceted stone. Preferences also vary with different cultures and religions, and the world's best opal stud earrings are cut in both styles. Don't make any inferences about quality based on cut style alone.
Cabochon. A facetless cutting style that produces smooth rather than faceted surfaces. These cuts can be almost any shape. some are round with high domes; others look like square-shaped domes (the popular "sugar cube" cabochon); others are "buff-topped", showing a somewhat flattened top.
Some connoisseurs believe that cabochons produce a richer color. Be that as it may, today we are seeing much more interest and appreciation for cabochons around the world and more beautiful cabochons than have been seen on the market in many years.
Faceted. A style of grinding in which we give the stone many small surfaces at different angles to each other, as in the various diamond cuts. The placement, angle and shape of the faces or facets are carefully planned and executed to show the inherent beauty of the stone; fire, color, shine, to the maximum advantage. Today, there are many faceted styles, including the "fantasy" cut, which combines rounded surfaces with shaped backs.
Importance of cut
The cut and proportions of colored stones are important for two main reasons:
They affect the depth of color seen in the stone.
Color and cut are the most important criteria in determining the beauty of a colored stone, followed by carat weight; higher carat weight will usually increase the price per carat, generally in a non-linear ratio. If the colored gem was a quality material to begin with, a good cut will maximize its natural beauty and allow it to display its finest color and vibrancy. If the same material is cut incorrectly, its natural beauty will be reduced, causing it to appear dark, too light or even "dead".
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