For our purposes, the word “overtone” will refer to a color or colors in addition to the main color of a stone. For example, most rubies exhibit colors in addition to red. The additional colors may be purple, pink, orange or brown. There may be one of these colors or several and they may vary as to what percentage they make up of the total observable color. There may be a great deal of purple and very little orange. In such a case the predominant overtone is purple and the secondary overtone is orange. In most gem varieties, very pure colors are usually the optimum. However in other varieties the overtones are considered to be attractive and will not cause a discount and may even bring a premium. Learn to read the overtones of the gems at which you look. It will help you become more sensitive to color and learn to evaluate the worth of stones more easily.
“Overtone” = the ancillary colors observable in a stone that modify the main color.
The term “body color” used in the gem trade refers to a gem’s color without reflection. In many cases the monetary worth of a given gem can best be determined by assessing the body color. In a windowed stone this can be done by looking down through the table into the area minus reflection. The color you see in that area is the “body color.”
In a more hopeful situation, with a well-cut stone we can determine “body color” by turning the stone upside down. When turned over, gems do not exhibit as much in the way of reflection or refraction; the color you see being abstracted from other optical properties of the gem is the “body color.” Intense reflection or refraction can mask the true “body color.” Determining body color helps a potential buyer in his dollar evaluation.
“Body Color” = the color of a gem when viewed without reflection or refraction.
Another word from the existing gemological lexicon is “pleochroism.” A pleochroic stone exhibits more than one color or shade of a color quite distinctly. As opposed to the way diamonds disperse light into colors, pleochroic stones show more than one color even when the colors seen are neither reflected nor refracted colors. The crystalline formation of a particular mineral causes pleochroism in cut gems.
We can break pleochroism into two distinct varieties– the most common being “dichroism.” For those familiar with prefixes, we know the “di” of dichroism means there are two colors in a dichroic stone. Commonly known dichroic stones are: ruby, sapphire, emerald, aquamarine, topaz, amethyst (quartz), tourmaline and peridot.
“Trichroism” is the second and less common form of pleochroism. For those of us who rode tricycles when young we know a trichroic stone has three colors. The gems falling into this category, while less well known, include alexandrite, kunzite, and andalusite.
Pleochroism must be distinguished from another type of phenomenon that displays more than one color. We call this phenomenon “color zoning.” A pleochroic stone will display more than one color no matter how many times it is cut. For example, if a 20 carat andalusite were cut into 20 one carat stones, each piece would display leochroism. In contrast, a color zoned stone such as a bi-colored tourmaline, can be cut in such a as to have two distinct pieces, each of a distinctly different color.
“Pleochroism” = exhibiting more than one color or shade of a color due to the crystalline structure of the variety of gem being examined.
Saturation as a concept has a particular interest to those people interested in gems. In a general sense, saturated colors are preferred in the gem world. The state of saturation is the state of being completely filled. The idea of being completely filled implies that no room exits for anything else. A saturated color therefore can be described as one which is both intense (completely filled) and pure (room for no other color). A saturated gem represents the optimum. A gem whose color is not saturated is discounted from that optimum. A case in point are the gradings given to fancy diamonds which in the case of yellow run: light fancy yellow, fancy yellow, intense fancy yellow. In addition, notations are made as to the overtones, if any, such as: brownish yellow, greenish yellow, or grayish yellow. The optimum for a yellow diamond occurs when the stone has the intense label and no overtones – a diamond with saturated color. Therefore, we can say a top grade yellow diamond is one which has a saturated color and be correct. We can also say a top grade blue sapphire is one which has a saturated color and also be correct.
One word of caution is necessary. There are gems whose color is too dark, especially in certain varieties of species, (i.e, green tourmaline, red garnets). The key to distinguishing between a nicely saturated color and a too dark color is a matter of practice and judgment. In the field of gemstones the use of the word “saturated” should be used to denote the state of intensity and purity of color combined.
“Saturation”= the state of being completely full of one particular hue.