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The refractometer is probably the most useful tool available to gemologists. It measures a gemstones ability to bend light or refract light. This measurement is known as the refractive index and it is specific for every gemstone variety. Doubly refractive stones will show both a high and a low refractive index (RI) while singly refractive stones will display only one RI. Refractive index tables are published in most gemology books and gemologists can look them up and compare results to identify gemstones.
An RI liquid (typically metheleyne iodide) and a monochromatic light source are required to use the refractometer. The liquid creates a kind of optical seal between the stone and the refractometer allowing the RI to be measured without any interference from ambient light. The monochromatic light source sharpens the RI line making it easier for gemologists to interpret the results accurately. |
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