Refraction
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A change in the velocity and direction of a light ray. In diamonds, the carbon atoms are so compact that light decelerates dramatically when it enters the stone. This allows a correctly-cut stone to reflect light and display the gem’s characteristic brilliance.
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Rhodochrosite
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A mineral whose colour ranges from rose to pink to almost yellow or brown. Although it is very pretty, this stone is soft and brittle; it is used in jewellery and for carvings and figurines.
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Rotor
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An unbalanced, semicircular pivoted mass inside an automatic or self-winding watch that swings with the movements of the wearer to wind the mainspring through a system of gears and ratchets.
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Ruby
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A precious stone that is a red variety of a transparent corundum, a mineral that is second only to a diamond in hardness.
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