Positions Of Optical Effects
CHECKING POSITION OF OPTICAL EFFECTS
To determine the strength, perfection, and position of any special optical effect, stand beneath a single pinpoint source of light, e.g., a single electric bulb (not a fluorescent light) or the sun. Diffuse light, such as that found out-of-doors on a cloudy day, results in broad, diffuse reflections which are more confusing than revealing.
As shown in picture, the stone is held below the eyes in such a manner that the rays from the light source pass close to the forehead. Tilt the gem slowly from side to side, noting how much the optical effect appears to tilt with each movement.
A few seconds’ experimentation will show whether or not the optical effect appears on top or appears to one side. Properly cut eye and star gems are shown in Figure 26. The same technique is used for moonstone, sunstone, and labradorite gems. It may also be used for precious opal, particularly black opal, and certain classes of white opal which tend to display their strongest colors along the edges of rough seam sections instead of on top, as indicated in Figure 27. Because cutting the gems across the narrow dimension of the seam will obviously result only in small gems, many lapidaries succumb to the temptations of greater weight recovery and cut their gems the wrong way.