Gemstone fashioning

The cutting and polishing of a gemstone is an important process because it will influence beauty and price. If well done, the faceting of a gem will display its beauty to optimum effect. However, if the cutting has been badly done, with little attention to correct orientation and proportions, the price and appearance will suffer. The objective in faceting a gemstone is to achieve beauty through brilliance, scintillation and dispersion. While many gems have little potential for dispersion, nearly all transparent gems can display brilliance and scintillation.

A faceted stone is designed to be a light trap - to collect as much light as possible - and throw it back to our eyes. Brilliance results when the proportions and angles are correct for the specific gem in question. This permits light entering the stone to be collected and internally reflected by pavilion facets and then forced to exit through the table facet.

Brilliance results from the combination of pavilion angle and table size - the pavilion traps light and reverses its direction; the table allows it to exit the stone. The single most important factor in producing brilliance is the pavilion angle. Depending on the gem's refractive index, it should lie within a fairly narrow range.

If the pavilion is too shallow, light leaks out through the bottom and a window results. If the pavilion is too deep, light passes out the side and extinction results. Both of these situations are examples of unplanned light leakage; one of the keys to good cutting is minimizing unplanned light leakage.

While brilliance is chiefly a function of good cutting, it will also be influenced by the stone's refractive index and transparency. As these two factors increase, so does the stone's potential brilliance. But a high refractive index is not a totally good thing for internal brilliance. Stones with a high refractive index will reflect a larger portion of the initial beam of light at the first surface. In a colorless gem such as diamond, this is fine, but in colored gems, the initial reflection (surface luster) can obscure the color. This is one of the reasons why gems with extremely high refractive indices often display rather grayish (low saturation) colors.

While the pavilion/table combination is responsible for brilliance, the crown facets add scintillation and dispersion. If the crown angle is too shallow and the table too large both dispersion and scintillation suffer. If the crown is overly steep, the stone loses brilliance and is more easily damaged when being worn. Thus the cutter tries to balance these factors to create the best overall combination of brilliance, scintillation and dispersion. With most colored stones, dispersion is not visible and so the cutter is concerned only with brilliance and scintillation.