DELIBERATE DEPARTURES FROM IDEAL ANGLES AND PROPORTIONS
During the above discussion the reader may have gained the impression that there can be no excuse for cutting faceted gems in other than correct angles and proportions. Actually, there are good and sufficient reasons for departing from the ideal as will be seen, especially when it is con¬sidered that most gems are meant for setting in jewelry.
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EFFECTS OF INCLUSIONS ON BRILLIANCE
One or two spots, specks, minute crystals, small cracks, etc., do not greatly affect the total amount of light which is returned to the eye in properly cut gems, although they may be disfiguring because perfect in¬ner clarity is established as the ideal for faceted stones. However, as the inclusions increase, they begin to scatter light, causing the rays to dart off at angles other than those desired. Many of these rays simply escape through the sides of the gems and contribute nothing to brilliance. Eventually the point is reached where so much light is lost that only a feeble glow appears on top, as in the jar din emeralds mentioned earlier.

Extreme losses occur in those gemstone materials which are only trans¬lucent, such as chalcedonies, for example, which consist of very many extremely small crystals grown together in compact masses. As the size of the individual crystals grows smaller and the spaces between them grow smaller too, light has less opportunity to be deviated from its path and more returns to the eye; such material is highly translucent, and sometimes, as in the case of williamsite serpentine, actually provides fairly satisfactory faceted gems.

In gemstones that are normally transparent, such as ruby, sapphire, and quartz, very small inclusions commonly occur which, though abundant, are too small to be seen clearly with the naked eye. Nevertheless, they seriously interfere with the pas¬sage of light and cause much scattering; a faceted gem cut from this sort of material can be a great disappointment to the lapidary who expected a brilliant gem for his efforts.

Other gemstones likely to have such inclusions are the blue sapphires of Kashmir, rose quartz, some heat-treated zircons from Thailand, and the green zircons of Ceylon. Sometimes the inclusions are so small that they cannot be seen under high-power mag¬nification; nevertheless, their presence is easily detected by the glowing, “sleepy” look of the finished gems.

REFRACTION AND ITS EFFECTS
The supreme considerations in transparent faceted gems are that the facets be placed at the correct angles to each other and that the finished gems have nearly the proportions shown in Figure 15. These angles and proportions were derived from many years of gem-cutting experience ac¬cumulated by lapidaries the world over and have been scientifically con¬firmed in modern times. The angles and proportions vary from one gem-stone species to another because the property inherent in every gemstone, known as refraction, or the ability of a gemstone to reflect and bend light rays, also varies according to species. Each species has its own char¬acteristic powers of refraction which the jeweler and gemologist measure on a small instrument called the refraciometer when testing gems. The strength of refraction is given a number called the refractive index. The number is high in diamond, and low in opal. The effect of this differ¬ence in refractive power can be readily seen if two well-cut brilliant gems, one of diamond and one of opal, both clear, colorless, and flawless, are placed side by side. The enormous brilliance of the diamond far sur¬passes that of the opal.

Gems of lower refractive  powers,  such as opal,  quartzes  (amethyst, citrine, rock crystal, etc.), and beryls, must be cut somewhat deeper in the pavilion than those of higher refractive powers such as diamond, zircon, sphene, and strontium titanate. Thus, if one took two correctly proportioned round brilliant gems of the same diameter, one of diamond and the other of rock crystal, it would be apparent at once that the pavilion of the diamond was not as deep as that of the rock crystal. Lapidaries are well aware of the effects of refraction and take this propcrty into consideration while cutting gems, adjusting the proportions and angles to suit the refractive index of each gemstone.

Slight departures from ideal angles and proportions can be made with¬out serious harm to brilliance as shown in Figure 16, where the ideal brilliant cut for diamond and a modern variation are diagrammed. There are obviously substantial differences in them, yet if two diamonds of identical color and clarity are cut to the same diameter, one in the ideal cut and the other in its variation, it is virtually impossible, without special light-measuring instruments, to detect consistently which one is more brilliant.

In general, more liberties with angles and proportions can be taken with gemstones of higher refractive index than with those whose refrac¬tive powers are less. In the higher index gemstones there is a greater “reserve” of refractive and reflective power, and appreciable deviations from ideal cutting still provide excellent brilliance. This latitude narrows as the refractive index decreases, until for gemstones of low refractive index, the lapidary must be quite careful with angles and proportions if the finished gem is to look brilliant at all.

Drastic departures from proper angles and proportions result in equally drastic losses of light as shown in Figure 17. If the facets are angled too steeply, they fail to reflect the light within the gem and much is lost through the back facets of the pavilion. A gem cut too shallow produces a gray area in the center called a “hole.” A gem cut too deep results in a general loss of brilliance or a grayness; the gem is marked by feeble, rather than strong, reflections. Both of these effects are best observed by holding a gem underneath a good light and looking down upon the top.

FACETED GEMS
The effectiveness of faceted gems depends on the fact that the bottom part of the gem, or pavilion, acts like a mirror. The light falling on the top of the gem passes through the transparent material, strikes the pa¬vilion facets, and is reflected upward to return to the eye. The use of many facets causes a dazzling display of individual sparkling reflections which change and shift as the gem is turned; these result in the effect known as brilliance.

BASIC CUTTING STYLES
To achieve these reflective effects, two basic types of facet cuts are employed, the step cut (or emerald cut as it is also called) and the brilliant cut. Both are shown in Figure 15. From them, and from combina¬tions of both styles, stem all of the many variations that have developed. The step cut consists mainly of parallel rows of strip-like facets, or steps. It has been used most effectively for emerald and other colored gem-stones such as aquamarine, topaz, and tourmaline. It has also been used for colorless gemstones, including diamond and zircon, but not as fre¬quently as the brilliant cut. The latter is preferred for colorless or very faintly colored gems, because the greater number of facets in the brilliant cut creates more complex and, thus, more interesting reflections. In the case of the diamond, it brings out the property of dispersion, or the splitting of white light into its colored components, much like a glass prism splits a beam of sunlight into the vivid hues of the spectrum. Sometimes we see dispersion at work when a beam of light strikes a cut-glass vase or pitcher, or fleetingly, we see it along the beveled edge of an old-fashioned plate-glass mirror. In diamond, this optical property I is very strong, and if the gem is faceted correctly, it results in numerous bits of vivid color flashing from the gem when it is held beneath a good light and turned in the fingers. It is for this reason that most persons are attracted to well-cut brilliant diamonds rather than to step-cut dia¬monds, which produce a less spectacular dispersion.

The two most common facet gem cuts, the standard brilliant and the
step cut.

Another important consideration in choosing between a step or bril¬liant cut is the need to reduce or disguise the disfiguring reflections which arise from inclusions or defects within the gem. In general, very slight flaws can best be hidden in the general dazzle of reflections aris¬ing from the brilliant cut rather than in the relatively few strip-like reflections arising from the step cut. A notable exception to this rule is in the emerald. When it contains so many small, evenly-spaced inclu¬sions, called jardin, or “garden,” that no style of cutting will disguise their presence, the step cut is considered acceptable, although, of course, the brilliancy of reflections is greatly reduced and an attractive “glow” supplants the normal, sharp reflections.

Faceted and Cabochon Gems

In this category you can learn more about two major types of cut gems: faceted gems and cabochon gems. Faceted gems are covered with small flat surfaces called facets, while cabochon gems feature a top surface of smoothly rounded shape, and a bottom surface which is usually, but not always, flat. There are many variations in both types as will be seen.
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