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With gems, we are dealing with opaque, matted finish object of uniform color. Thus it is not enough to just describe hue position, saturation, and darkness. It is also necessary to describe color coverage, scintillation, and desperation.
Color coverage can be influenced by a variety of factors, including proportions, fluorescence, and inclusions.
The round red spinel at left is strongly fluorescent and red emission adds extra power to the red body color, covering up extinction. The fine emerald cut sapphire pictured at right, color coverage is improved by the presence of tiny needle like inclusions, which scatter light across the stone, thus reducing extinction. This is what gives the sapphire its incomparable velvety color. Note that both of these gemstones have colors which are highly saturate, making them highly desirable.
Color Coverage
Differences in inclusions, transparency, fluorescence, cutting, zoning, and pleochroism can produce vast differences in the color coverage of a gem, particularly faceted stones. A gem with a high degree of color coverage is one in which color of high saturation is seen across a large portion of its face in normal viewing positions. Tiny light scattering inclusions such as rutile silk, can actually improve coverage, and thus appearance, by scattering light into areas it would not otherwise strike. The end effect is to give the gem a warm, velvety appearance (Kashmir sapphire are famous for this). Red fluorescence in ruby boosts this still further.
Proper cutting is vital to maximize color coverage. Gems cut too shallow permit only short light paths, thus reducing saturation in many areas. Such areas are termed windows. Those cut too deep allow light to exit the sides, creating dark or black areas termed extinction. Areas which allow total internal reflection will display the most highly saturated colors. These are termed brilliance.
Color zoning can be influenced by an unevenness of color. The oval sapphire at right shows moderate color zoning. Moderate to severe color zoning does impact quality and value. Color zoning is always judged in the face up position, in an 180 degree arc from girdle to girdle, with the gem rotated through 360 degree. Color irregularities visible only through the pavilion generally do not impact value.
Color zoning can also reduce color coverage. Ideally, no zoning or unevenness should be present.
Pleochroism is sometimes noticeable in ruby and sapphire. It typically appears as two areas of lower intensity and/or slightly different hue on opposite sides of the stone. This is most notable when the table facet lies to the c axis.
To summarize, a high quality gemstone display the hue of maximum saturation across a large percentage of its surface in all viewing positions. The closer a gem approaches this ideal, the better its color coverage.
Scintillation (Sparkle)
An important factor in faceted gemstones. A gem stone 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 reflection can not be individually distinguished by eye, resulting in a blurred appearance.
Dispersion (Fire)
This involves splitting of white light into its spectral colors as it passes through two unparalleled surfaces , such as a prism. The dispersion of corundum is so low and the masking effect of the rich body color so high, that it is generally not a factor in ruby and sapphire evaluation.
When examining a color gemstone a plain white background is the best.
Clarity
Clarity is judged by reference to inclusions. Magnification can be used to locate inclusions, but the exception of inclusions which might affect durability, only those visible to the naked eye should influenced the final grade. There are two factors in judging clarity. These are:
Visibility
Size: Smaller inclusion are less distracting, and thus , better Number: The fewer the inclusions, the better Contrast: Inclusions of low contrast, compared with the gem’s color, are less visible, and thus, the better Location: Inclusions in locations near the girdle rather than directly under the table facet affect value less. Similarly, a feather perpendicular to the table is less likely to be seen than one lying parallel to the table.
Affect on durability
A) Type: unheated cracks may not only be unsightly, but also lower a gemstone’s resistance to damage. They are thus less desirable than a well heated fracture. As already mentioned, tiny quantities of exsolved silk may actually improve a gem’s appearance, and thus, value.
B) Location: A crack near the culet may or corner would obviously increase the chances of breakage more than one well into the gemstone. Similarly, an open fracture on the crown is more likely to chip than one on the pavilion.
Among the problems of existing color gemstone grading system is that the model chosen is based on diamond. While diamond does share a number of quality factors with ruby and sapphire, others are partly or wholly inappropriate. For example, beauty in diamond is largely a function of the stone’s brilliance and dispersion (fire). Any inclusions which alter the path of light could be detrimental to a diamond’s appearance. Perfect clarity is thus the ideal. As described before, perfect clarity is not necessarily the ideal for ruby and sapphire. While fractures and most other inclusions do have a detrimental effect on appearance and durability, small quantities of finely dispersed inclusions, such as exsolved rutile silk, can actually improve a richly colored gemstone’s appearance. The watch word here is small, too much silk decrease transparency by scattering , reducing color saturation, and thus producing a more grayish color.
Below, two types (Table 6) of GIA clarity are shown:
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