Q: In a faceted gemstone, what characterizes a good or a bad cut? Can a transparent gem be set in a dark depression and still sparkle? Does it need light from the sides or bottom?
A: Overall, the shape should be symmetrical and precise, with outlines that are doing what their geometry suggests they should. Ovals should be oval, not lumpy blobs. Rounds should be round, not almost so. Squares should be square, and parallel sides should not be just that little hair out of parallel. In short, the shape and overall cutting of the stone is best when it looks like the cutter actually managed to cut it correctly into the shape he had in mind, without compromises for weight retention and misshapen rough.
Facets should be flat, well polished, and meet properly at clean points and corners. They should be symmetrical, with facet shapes uniform and balanced all around, the table centered, culet centered, and a crisp overall look. The crown of the stone should have enough height, with a small enough table (but not too small), so that the light is attractively refracted and “mixed up” This creates the sense of movement and glitter when you move the stone, the light, or your eye, that we call scintillation. In a stone which has enough dispersion to exhibit fire or dispersive colors, then the crown angles must be such as to take advantage of this. (e.g. diamonds or cubic zirconias) Faults in the crown are usually shallowness, too large a table, or too high and lumpy a look.
The pavilion of the stone must be cut at the correct angles to allow light to reflect back to the eye. Actually, light enters the top, hits one pavilion surface, reflecting horizontally through the stone to the opposite pavilion surface, and then back to the eye. This double reflection is what allows the stone to be “brilliant”, which refers to the ability of a faceted stone to reflect light shining into the top back out the top to your eye.
The higher the refractive index of the stone, the better will be the potential for a stone to correctly, completely, and uniformly do this little trick. Stones with low refractive indexes may have trouble being completely brilliant, but they should at least try. And in some of the fancy shapes, marquise and pear shapes especially, it will be quite difficult to get this brilliance all over the stone.
Faults usually amount to overly shallow pavilions, which let you see through the stone to what’s underneath—this is called “windowing”—or overly deep pavilions, which just look dark—this is called “extinction”. Some pavilions will be cut with both effects, due to a large degree of bulge, or a difference between the pavilion angle at the girdle, where it’s steep, to that at the culet (different row of much shallower facets) where it can be too shallow. This bulge, however, can also be used judiciously, with colored stones, to allow a correct pavilion angle at the culet, but a slightly deeper overall pavilion depth. In some colored stones, this can result in intensified color. So there’s no single hard and fast rule. The idea is that the cutter should maximize the potential beauty of the stone, which means both color depth and intensity, as well as brilliance.
As to setting stones in closed backed settings: In theory, if a stone is properly brilliant, then virtually all the light you see coming out the top is light that entered the top, and has reflected back to you. In this case, a closed back will have little effect on the look of the stone. This is most true of high refractive index stones, such as diamond. A corollary to this is simply that the old story about stones needing light from below is generally not true. Light that enters most stones from the side or below usually does not exit out the top of the stones, even with poorly cut ones.
But there’s a big catch to all this. The double internal reflection that light must achieve in a well cut stone depends very much on the exterior surface of that pavilion being clean. Dirt or grease on that surface destroys the ability of the stone to reflect light. If a setting is closed in the back, then it becomes much, much more difficult to clean the stone properly. Real life being what it is, It’s very unlikely that any stone will be set such that the back is completely sealed from dirt. So over time most stones, even with sealed backs, will get dirty and need cleaning. With a closed-back setting, you can be out of luck.
Obviously, there are exceptions. The principal one I’m aware of is that some of the class ring manufacturers use closed-back settings under the synthetic faceted stones in their rings. The key to these designs staying clean is that the stones are actually set against a plastic gasket, not against a metal bezel. That gasket provides a sufficient seal against dirt. Of course, it also provides a nightmare for any service such as sizing the ring. Even modest heating, in what might otherwise be a stone that could easily withstand some heat, will quickly destroy those plastic gaskets.
The other exception is in the other direction, and is the case of stones cut to very poor optical proportions, such as very shallow ones, with much windowing. Obviously, if the stone shows windowing, then you can see what’s behind and under the stone. So then whether the setting is closed-back or not can dramatically affect the look, as you see the setting through the stone. Closing the back of the bezel in such stones usually leads to a darker look, while opening the sides or bottom can make it look lighter. In these cases, details such as how the inside of the setting is polished or not can have a dramatic effect on the overall appearance of the finished work. It then is up the jeweler to decide which combination of setting style, background finish, and other variables will give the best effect.
The tradition in setting faceted stones is to have the bottom opened up. In fine work, this will even be a flared and carefully shaped opening, so the appearance from the inside or back of the setting is as carefully refined and considered as that from the front. Often a saw or gravers are used to shape the underside openings so they are a series of square flared openings in a line (for stones set in a line), or other shapes which equally fit together reflecting the stone arrangement on the front. Unneeded metal is removed, leaving at the back only a web of narrow ribs separating the stones from each other. This is called azuring the setting. A well-azured setting is the mark of a skilled craftsman. Taking time and skill to do well, it can increase the value of a piece of jewelry. It not only looks impressive, but it makes the stones easier to clean and service.
When azuring techniques were originated, most stones were generally cut poorly with regard to windowing and brilliance, so the technique then made a lot of difference to the appearance from the front. Nowadays, especially with diamonds, the proportion of poorly-cut stones is much less, and designers are freer to choose whether and how to open up the back of a setting. But in general, it’s considered poor form to completely close a setting for a transparent stone. At the very least, it should usually be drilled through. So if a designer has chosen to close the back of a setting, it must be for a valid and good reason, not merely laziness on the part of the setter or poor judgment on the part of the designer, and the need to clean the piece in the future must somehow be addressed.