The determination of beauty and tone are important points in judging the quality of gemstones. The quality scales used in this book are arranged with beauty along the horizontal axis and tone on the vertical axis.
Tone is divided into levels ranging from 7 to 1, with 7 being “dark”, 5 “medium”, 3 “light”, and 1 “extremely light”. Data from spectrophotometric readings were sampled, and the final tone levels were established visually. Whereas the beauty scale represents a complex combination of a variety of factors, tone is a linear scale dealing with just one factor.
Color can be thought of as being comprised of three components: hue, which is represented by the basic color sensations of red, orange, yellow, green, blue, and violet-such as the red of ruby, the yellow of citrine, or the blue of sapphire; saturation, which indicated whether a color is a beautiful pure color, and the degree of gray or brown overtones present; and tone, which describes the color’s level of lightness or darkness.
A gemstone’s tone can be accurately judged by placing it face up and comparing it to master stones. It is important to make the comparison under a variety of conditions, such as placed on a piece of white paper or between two fingers. In any case, master stones are the key to accurately determining the tone of a gem. It is easiest to judge tone with master stones similar in shape, cut, proportions, and size to the gemstone being evaluated, but it would be impractical to have master stones for each different gemstone being tested. In reality, the same master stones can be used to evaluate the tone of gemstones of different colors. However, since the tone and the mosaic pattern appear differently depending on a stone’s size, it is necessary to have separate master stones for large and small stones. |
|
When judging faceted gemstones in the face-up position, one must look at a three dimensional tone that includes such factors as the mosaic pattern’s balance, the effect of reflections, and how much light enters the stone. It is possible to further differentiate between high, medium, and low tones within a single tone level, but normally there is no reason to make such a fine distinction. Yet in the cases of gem qualities of sapphires larger than 5 carats, rubies larger than 3 carats, and diamonds larger than 1 carat, tone levels will be precisely judged when establishing value.
When judging a diamond’s color grade, the tone of fancy-colored diamonds is graded face-up, and colorless to light yellow diamonds of tone levels 2* and lower are graded from the side, as shown below. This allows more accurate and objective judgment of the tone by minimizing the influence of the brilliance and dispersion caused by the faceted cut. Grading the range from D to Z requires distinguishing minute differences in tone levels. When looking at diamonds set in adornments, however, adjacent steps on the D-to-Z scale are indistinguishable. Differences in tone should be reflected in value of only where they are clearly visible in a stone’s appearance.
The tone of a gemstone also affects the way inclusions appear. Depending on its nature, even a fairly large inclusion that is obvious in a light stone may not be very noticeable in a darker one. The use of inclusion size as an objective yardstick in evaluating quality becomes meaningless if one fails to recognize the relationship between inclusions and tone levels. More importantly, one must decide if the inclusion is an acceptable inclusion that serves as a proof if natural origin, or a defect that threatens durability. Even if a gem is dark, making inclusions difficult to see, an expert must inspect the stone with a loupe, looking out for fractures and defects that threaten durability. |
Demand will focus on the tones that are traditionally preferred. Assuming that the supply of gemstones is spread evenly across the various tone levels, gems with these preferred tones will become more rare and have higher value. For each gemstone type, factors such as the properties of the gem, limitations of supply, and traditional preferences will result in differences in what tone levels are most highly valued, such as tone 0 for diamonds, 3 for aquamarine, and 5 for ruby. Also, there are stones that are attractive in light tones and those that are not. Since cabochons do not have the beauty of a mosaic pattern’s brilliance, they cannot fully exhibit their beauty without a certain degree of darkness in their color. As circumstances cause supply and demand to fluctuate, the range of preferred tones may also shift, resulting in changes in value. Popularity and the discovery of new mines will bring about shifts in the marketplace.
|
|
