||All gem testing polariscopes are operated on the same principle. Gems are examined between crossed polaroids; i.e., with the vibration direction of the analyzer turned at right angles to that of the polarizer. This is the dark, or extinction, position. When the upper Polaroid has been turned to the position of minimum light passage, the stone is placed between the two plates and rotated. A stone to be examined in the polariscope may be held in stone tweezers or in the testerís fingers. The polariscope can be used effectively for transparent or translucent gemstones. Since it analyzes transmitted light, it will not work for opaque materials.
If the stone darkens and becomes light upon each 90 degree rotation, double refraction is indicated. If it remains dark, it is singly refractive. In a doubly refractive stone the change varies from an abrupt light to darkness across the whole stone to one in which only a dark band move across the transparent stone as it is rotated. The difference depends on the relationship of the testerís angle of view to the optic axis of the stone. Closer to the optic-axis direction, a band of dark moving across a usually light stone is to be expected. Within a few degrees of parallel to the optic axis, colors are usually visible in a doubly refractive stone.
The Illuminator polariscope uses a single casting to mount a lamp and the two Polaroid plates. The lamp is housed in the base, beneath the lower polaroid plate., The upper polaroid plate is rotated in a plane parallel to the lower polaroid. It is held between three and four inches above the polarizer on an arm extending over the lower plated. A light portal at the front of the instrument serves as a light source both for the refractometer and the dichroscope.