Do the 4Cs for diamonds apply to colored gemstones?
Yes and no. They provide a useful basis for examining and comparing stones. But they’re nowhere near as precisely measurable. As a result, there isn’t a uniform grading system for colored gemstones like there is for diamond quality. Diamond grades are, in essence, light-performance ratings. They tell you how much light is reflected back to the eye and how free of yellow or brown tinting it is.
Gemstone color
Gemstones are far harder to grade for color. Evaluating gem color involves the interaction of three interactive factors: 1) hue, 2) tone and 3) saturation. Hue refers to the basic color composition of a stone. It is usually noted in terms of primary and secondary spectrum colors. For example, a ruby’s primary color is red with varying degrees of secondary pink or purple; a sapphire’s primary color is blue with varying degrees of secondary violet and green; an emerald’s primary color is green with varying degrees of secondary blue or yellow. Tone refers to the lightness or darkness of a stone on a scale starting at faint or washed out and running to opaque or pitch black. Most desirable gems have a medium tone, although medium-dark and medium-light tones are acceptable, depending on the gem species. Saturation refers to the purity and vividness of primary color. As stones incorporate increasing amounts of color-suppressing gray or brown, their hues become softer, more subdued, losing drama and punch. However, this decrease in intensity of primary color is not necessarily a negative. Grays can soften blues into delicate pastel shades and browns can deepen reds into rugged autumnal colors. Gem professionals judge how these three color factors interact to create a pleasing color. Although rarity and standards for each variety do affect value, color grading of colored gemstones is largely about beauty.
Gemstone clarity
During the last few decades, quality standards once sought only in diamonds have become generalized to colored stones. As this has happened, visible flaws have become less forgivable in colored stones. To give inclusion-filled colored stones, like emeralds, the immaculate appearance expected of diamonds, gem dealers increasingly rely on gemstone enhancement to erase or camouflage visible inclusions. That’s the only way the jewelry industry can indulge the public’s growing intolerance of included gems. There’s a better way: Be realistic. Don’t expect all colored stones to have the eye-clean clarity of diamonds. In fact, the GIA says clarity expectations should be determined by the gem you are considering. Some gems are relatively inclusion-free while others are normally included. It may be asking too much of certain gems to expect them to be “eye clean.” To prevent unrealistic clarity expectations, GIA has even devised a 3-tier system of clarity ratings consisting of Type I, II, III. If you’re looking at Type I stones, your expectations can be justifiably high; less with Type II and even less with Type III. Type I gems include aquamarine, tanzanite, and chrysoberyl. Type II gems include ruby, sapphire, alexandrite, garnet and amethyst.
Type III, which are very rare without visible inclusions include emeralds, red beryls and red and pink tourmalines. In any case, the presence of one or two noticeable inclusions shouldn’t be an outright reason for elimination of a colored stone from consideration. Keep color-quality uppermost in your mind. As a rule of thumb, your comfort level with visible inclusions should depend, in part, on the general tones of the gem you are considering. That is why RockHer advises you to read up on any gem you are thinking of purchasing. For instance, flaws are easily spotted in pastel stones like aquamarine and understandably much more objectionable than in deeper-color stones like blue sapphire where they are harder to detect. However, certain gems with deep colors such as emerald demand greater leniency with regard to flaws because they are, by nature, more heavily included. Overall, it is impractical to have notions of colored stone clarity modeled on those for diamond. Remember that desirable diamonds are colorless and transparent. Hence visible flaws are unsightly and to be avoided. But visible flaws in gems desired primarily for their color should be far more acceptable--if only because they are far more common.
Gemstone cutting
The objectives of colored stone and diamond cutters are usually the opposite. Diamond cutters strive for full and instantaneous return of light to the observer’s eye. So they cut for the quickest light-processing time possible. With them, it’s in and out. Colored gemstone cutters strive for fullest return of color to the observer’s eye. So they cut for much longer light-handling time so that stones can pick up as much color as possible. With them, it’s stay inside a while. Nevertheless, colored stone and diamond cutters share some concerns--especially with the weight of stones.
Gemstone carat weight
It is better to buy gemstones for their color and not their carat weight. When a colored stone is cut for maximum color strength, it will stun with sizzle rather than impress with size. Since colored stones are not sold, or categorized, by the exact weight, the per-carat-price discounts for buying stones just under a size-category do not automatically translate into significant percentage discounts.
When stones are cut more for bulk than beauty, and, for example, given needlessly buxom bottoms, they hemorrhage light out the sides. There are two immediate visual consequences from gem obesity: 1) windowing and 2) extinction. Stones that are windowed look more like glass than gems. You can often read a newspaper placed underneath them. Stones with heavy extinction act like black holes and keep too much light inside the stone. The more undesirable windowing or extinction a stone has, the less of the most desirable quality it will have: brilliance. When stones are well-cut, brilliance should account for at least 80% of its cutting quality. Brilliance is never 100%. There will always be a little bit of extinction. When controlled, tiny black areas of extinction can give strong color contrast and boost the visual impact of a stone. But past a certain point, it eventually becomes a blockade to color.