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The polariscope is a useful tool which helps gemologists identify gemstones. All gemstones are either doubly or singly refractive and the polariscope is the simplest way to determine this important optical property. Polariscopes employ a set of parallel polaroid plates mounted at a sufficient distance apart to permit gemstones to be examined between them. One plate is known as the analyzer and one is known as the polarizer. The plates are fixed but at least one of them must be rotatable.
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.
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