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Refining - purifying metals..
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Refraction - the bending of light as it passes through a gemstone.
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Relief - the contrast between the inclusion and the gemstone. A good example of this would be a lower clarity grade diamond that has eye visible inclusions (the internal characteristics).
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Rhodium - is a metal in the platinum group. It is often used as a platinum alloy. Its purpose is to harden the platinum to give it greater durability for jewelry. The most common percentage mix of rhodium in jewelry is 95% platinum and 5% rhodium. It provides a non-tarnishing electroplate with high reflectivity. It gives the gold a high bright shine when polished.
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Rough - an uncut gemstone.