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 tester’s fingers. The polariscope can be used effectively for transparent or translucent gemstones. Since it analyzes transmitted light, it will not work for opaque materials.