Just like diamonds, the value of colored gemstones is determined by color, clarity, cut and size (carats). With precious colored gemstones, such as ruby, sapphire and emerald, which already command a high price because of their rarity, subtle variations in any of the quality factors will mean a significant difference in value.
Almost all other gemstones are categorized as "semiprecious." With semiprecious stones, the same quality factors determine the grade, but each factor may be weighted differently depending on the rarity of that feature for the particular type of stone.
Colored gemstone grades range from AAA to D. The AAA, AA and A ratings, known as "fine grade," are reserved for exceptional colored gemstones that demonstrate top quality in all quality factors. Grade B gemstones tend to have some visible, undesirable inclusions, the color is not optimal, and they generally have less sparkle. However, grade B stones are used by some jewelry designers because they are less expensive than Grade AAA-A stones. Grade C and D stones have significant to heavy amounts of visible inclusions and a dull appearance. Although they are significantly cheaper than grade B or better stones, grade C and D stones are not used in high-quality jewelry, since they are noticeably less attractive.
With semiprecious gemstones, color is usually, but not always, the main consideration in determining the stone’s grade. Generally speaking, the more pure, rich and intense the color, the more desirable and valuable a gemstone will be. An example is natural lapis lazuli, a relatively rare semiprecious stone that has been prized since antiquity for its intense blue color.
For those semiprecious gemstones that come in many colors, color may not be the most important factor driving the grade and price. For example, tourmaline comes in almost every imaginable color, so unless the color is an exceptionally pure, intense pink or green, all colors of tourmaline are considered to have about the same value. The other factors of clarity and cut therefore will be more heavily weighted in determining the value of a tourmaline stone.
Similarly, for colored gemstones whose natural hue, saturation and tone are commonly lighter, color will not be weighted as strongly when grading the stone. These lighter gemstones are generally more transparent or translucent, which makes the other factors of clarity and cut more noticeable, and therefore more important in valuing the gemstone.
Gemstones contain a wide variety of natural inclusions, ranging from colorless, bubble-like or hazy areas to colored specks or cracks, which can affect how transparent or durable a stone is. Some inclusions actually add to the beauty and increase the desirability of certain gemstones, such as the appearance of mineral-based rutiles — the colored needle-like projections — found in tourmalated quartz and rutilated quartz.
However, the value of many colored gemstones is diminished by the presence of inclusions, especially when those inclusions are so numerous that they interfere with the passage of light through the stone and make it look cloudy. Some inclusions are actually internal cracks, which affect the durability of the stone.
The cut of a gemstone is one of the most important factors in its appearance. A gemstone that is cut well will have more brilliance and life than a poorly cut stone of the same size and color. Still, there are a wide variety of styles and shapes for colored gemstones. Like diamonds, some gemstones are cut in a symmetrical shape and highly polished for a more formal appearance. Other gemstones are cut in less-structured shapes for a more organic appearance.
Regardless of the shape, certain factors indicate a well-cut gemstone. If faceted, facets should be crisp and come together in a single point. Lower-quality stones have facets that are slightly rounded and which do not quite meet where they should, which will affect the brilliance of the gem. All areas where light is reflected from the surface should be smooth and mirror-like. Pits, scratches or dull areas are indications that the gem is not well-polished.
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