COLOR AND COLOR DISTRIBUTION
In many gemstones, color tends to be uniformly distributed in the rough, and the lapidary’s only concern is that of cutting the largest gem consistent with the shape of the rough. In other gemstones, however, the color may be unevenly distributed or even changeable in character from place to place as shown in Figure 5. Amethyst is frequently an offender, with its color confined to narrow bands with colorless material between. If these bands appear edge-up as one looks down upon the gem, they will be easily noticeable and detract from the appearance. The skilled lapidary carefully notes their position and orients the rough in such a manner during shaping that the bands will lie approximately parallel to the top surface of the gem. Thus if one buys amethyst rough, it is neces¬sary to look for this defect and select only those pieces in which it will be possible to place the bands correctly and still obtain a good yield.
Sometimes color occurs in only one spot or area, the rest of the rough being pale in hue or even colorless. If the rough is shaped as shown in Figure 6, with the spot of color placed at the apex of the pavilion, the finished gem will be “flooded” with color and could appear quite hand¬some.

A particularly troublesome color problem arises in gemstones which show marked differences in color depending upon the direction within the crystal. When two colors appear, the effect is called dichroism and may be present in such gemstones as tourmaline, beryl, apatite, benitoite, ruby, sapphire, and zircon. The effect is often so strong in tourmaline crystals that it can be easily detected with the unaided eye. The dichroic effect is shown diagrammatically in Figure 7. Very commonly, the color along the length of the crystal, or the direction of the optic axis, is a disagreeable shade of olive which is sometimes so dark that it seems black. However, looking through the sides of the crystal, the color is a much more pleasing shade of yellow-green. Figure 7 shows how a step-cut gem must be placed in such a crystal if the final color is to be pleasing. In some dichroic gems the lapidary reverses the procedure to obtain the best “face-up” color. For example, the best color in a pink tourma¬line is seen looking down along the crystal axis instead of across; thus the gem has to be cut so that the top facets lie across the crystal instead of parallel to its length. Other dichroic gemstones that require this ori¬entation are ruby and sapphire and some beryls. To be certain of the proper orientation, some cutters use a small instrument called a dichroscope which separates the two colors clearly and enables the skilled user to tell which direction in the rough gemstone would produce the best results. One of the simplest of these instruments is made from pieces of polaroid film; its use is shown in the photograph actual crystals of pink tourmaline are being tested. The differences in color shade are apparent in the two windows of the instrument.
In still other kinds of gemstones, three colors occur in the crystals according to direction, and the effect then is given the name trichroism. It is more difficult to orient a gem properly in such crystals, but again the dichroscope is helpful. The cutter examines such crystals with care to observe the separate colors. Only two colors can be seen at one time, but by turning the crystal about he eventually detects the third color. He then makes a choice of direction based upon the most pleasing and strongest color, remembering that now at least two of the three colors will blend into a combined hue. Some experience is necessary in the cutting of such crystals, but once the tricks have been learned it is not a difficult procedure. Striking trichroic colors are seen in iolite (pale yellow, pale blue, deep blue) and in facet-grade andalusite (yellow, green, red).
It now only remains to say that in a third class of gemstones there is no color difference according to direction and of course no inspection of the rough is required with the dichroscope, the lapidary needing merely to look for possible patchiness or zoning of color. Some gemstones in this class are garnet, opal, natural glasses (tektites), obsidian, and spinel. Those gemstones that would normally show dichroism or tri-chroism but are composed of a great many minute interlocked crystals also behave in this fashion, e.g., jadeite and serpentine. If highly translucent specimens of these gemstones are held before the dichroscope, no color changes will be detected.
The table on page 39 shows the principal gemstones exhibiting pro¬nounced dichroism or trichroism. Strong color differences require proper orientation if satisfactory gems are to result.