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Although there is no industry standard for grading gemstones, almost all agree that gemstones are characterized by the following four defining attributes: color, cut, carat weight and clarity.
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A gemstone's unique coloring is determined by its tone, hue and saturation. Generally speaking, the more intense and evenly distributed a gemstone's color, the higher the value of the stone.
Tone: The tone of a gemstone refers to the deepness, richness, or darkness of the stone's hue. A gemstone's tone can range from "very very light', or virtually colorless, to 'very very dark', or almost completely opaque. A quality stone is transparent enough to maintain light return but dark enough to show a richness of color.
Hue: The hue of a gemstone is defined by the specific shade of color it exhibits. The more unified the shade, the higher the value of the stone. A gemstone that looks entirely blue rather than bluish-green would be considered a higher quality stone. Aside from Opals, gemstones that display two or more colors that compete for the eye's attention are considered to be of lower value.
Saturation: Saturation refers to the intensity or purity of a gemstone's color. An impure color may appear dirty, muddy or cloudy. Typically stones with a higher level of saturation display brighter, more vivid colors.
The cut of a gemstone, like the cut of a diamond, enhances its overall beauty and value. A good cutter wants to capitalize on valuable carat weight, color, clarity, and symmetry. Because gemstones can vary so greatly in color and raw carat worth, there are no standard measurements or proportions for gemstone cuts. The quality of each cut must be judged according to the quality of each stone.
Carat Weight: Gemstones, specifically from the corundum family, such as rubies and sapphires, are significantly more expensive than other gemstones. For this reason, a cutter may choose to keep an inclusion in favor of producing a stone with a higher carat weight.
Clarity: The clarity of a gemstone refers to scratches, fissures and other inclusions which might detract from the overall beauty of the stone. A quality cut will remove as many jarring inclusions as possible and will exhibit a smooth, scratch-free polish.
Color: Depending on the color of the stone, the proportions defining a quality cut will change. A stone with a dark color, for example, should be cut with a shallow pavilion to brighten its overall appearance. A light-colored gemstone, on the other hand, may benefit from a deeper cut.
Symmetry: The facets of a gemstone should always be symmetrical. An asymmetrical cut not only looks sloppy, but also affects light distribution and reflection across the stone. Well-cut gemstones are designed to evenly distribute light.
All gemstones have some traces of minerals, cracks, or fissures, referred to as inclusions. However, many gemstone inclusions do not negatively impact the value of the stone. Because there is such a wide variety of gemstones, there is no universally accepted grading system for gemstone clarity and every stone's clarity must be measured according to its own variety. Naturally, gemstones with very few to no inclusions are considered to be the most valuable and may price higher than high quality diamonds due to their rarity.
The weight of a gemstone isn't necessarily correlated with its size. Gemstones vary considerably in density so that a one carat ruby will be far smaller than a one carat opal. When buying a gemstone, Zoara suggests that you don't rely on the carat weight to give you a sense of the size of the gem or the size of the mounted gem. When it comes to evaluating gemstones, the weight of a gemstone, or its density, and not its size, will determine the gemstone's value. A gemstone that is denser, that weights more, will cost more than a gemstone that may appear larger in size, but has a lesser density.
Invented by German mineralogist Friedich Mohs, the Mohs Scale evaluates the hardness of a given material based on its scratch resistance. This idea was actually implemented by the ancient Greeks as early as 300 BC, but Mohs was the first to create a reliable standard scaling system.
The Mohs scale includes ten minerals of various levels of hardness, the hardest material being 'diamond' and ranked '10' on the scale. In order to determine the hardness of a given material, one would simply find the hardest mineral which can be scratched by the material in question, and its hardness would be estimated just below the level of hardness of that mineral.