The function of the cut is to display the gem’s inherent beauty to the greatest extent possible. Since this involves aesthetic preferences upon which there is little agreement, such as shape and faceting styles, this is the most subjective of all aspects of quality analysis.
|The parts of a faceted gem. Photo: Wimon Manorotkul|
Evaluation of cut involves five major factors (in no particular order):
- Cutting style
Shape. This describes the girdle outline of the gem, i.e. round, oval, cushion, emerald, etc. While preferences in this area are largely a personal choice, due to market demand and cutting yields, certain shapes fetch a premium. For most colored stones, ovals and cushions are the norm. Rounds and emerald shapes are more rare, and so receive a premium from about 10–20% above the oval price. Pears and marquises are less desirable, and so trade about 10–20% less than ovals of the same quality. The shape of a cut gem almost always relates to the original shape of the rough. Thus the prevalence of certain shapes, such as ovals, which allow greatest weight retention.
Proportions. The faceted cut is designed to create maximum brilliance and scintillation in the most symmetrically pleasing manner. Faceted gems feature two parts, crown and pavilion. The crown’s job is to catch light and create scintillation (and dispersion, in the case of diamond), while the pavilion is responsible for both brilliance and scintillation. Generally, when the crown height is too low, the gem lacks sparkle. Shallow pavilions create windows, while overly deep pavilions create extinction. Again, proportions often are dictated by the shape of the rough material. Thus to conserve weight, Sri Lankan material (which typically occurs in spindle-shaped hexagonal bipyramids) is generally cut with overly deep pavilions, while Thai/Cambodian rubies (which occur as thin, tabular crystals) are often far too shallow.Cutting style. The cutting style (facet pattern) is also a rather subjective choice. Again, because of market demand, manufacturing speed and cutting yields, certain styles of cut may fetch premiums. The mixed cut (brilliant crown/step pavilion) is the market standard for ruby and sapphire, while the step (emerald) cut is the standard for emerald.
- Depth percentage: In attempting to quantify a gem’s proportions, reference is often made to depth percentage. This is calculated by taking the depth and dividing it by the girdle diameter (or average diameter, in the case of non-round stones). The acceptable range is generally 60–80%.
- Length-to-width ratio: Another measurement that is used for non-round stones is the length-to-width ratio. Overly narrow or wide gems of certain shapes are generally not desirable.
If a gem is cut too shallow, light will pass straight through, rather than returning to the eye as brilliance. This is termed a “window” (right). In well-cut gems, most light returns as brilliance (left). Brilliant areas are those showing bright reflections. Extinction is used to describe dark areas where little or no light returns to the eye.
Photo: Wimon Manorotkul
Symmetry. Like any finely-crafted product, well-cut gems display an obvious attention to detail. A failure to take proper care evidences itself in a number of ways, including the following:
Finish. Lack of care in the finish department is less of a problem than the major symmetry defects above, because it can usually be corrected by simple repolishing. Finish defects include:
- Facets do not meet at a point
- Misshapen facets
- Rounded facet junctions
- Poor polish (obvious polishing marks or scratches)
Scintillation (‘sparkle’). This is an important factor in faceted stones. A gem cut with a smooth, cone-shaped pavilion could display full brilliance, but would lack scintillation. Thus the use of small facets to create sparkle as the gem, light or eye is moved. In general, large gems require more facets; small gems should have less, for tiny reflections cannot be individually distinguished by the eye (resulting in a blurred appearance).
Dispersion (‘fire’). This involves splitting of white light into its spectral colors as it passes through non-parallel surfaces (such as a prism). While diamonds show this property to great effect, with most colored stones, their dispersion is too low and the masking effect of the rich body color so high, that it is not generally a factor. Exceptions are gems such as demantoid garnet, grandite garnet from Mali and sphene. In gems such as these, a weaker body color can actually be desirable, making the fire more visible.
Summing up cut. While these guidelines may be useful, one must not become a slave to them. In essence, the cut should display the gem’s beauty to best advantage, while not presenting setting or durability problems. If the gem is beautifully cut, things such as depth percentage or length-to-width ratio matter not one bit. What works, works. The eye, the mind and the heart are the final arbitrators, not numbers.
One final note about cut. The most expensive colored gems (particularly colored diamonds and rubies) often feature misshapen proportions and symmetry. This is because the value of the material is so high that the cutter strives to save every point in weight.