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Rhyolite

A fine-grained, extrusive igneous rock, having parallel mineral composition with granite

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Raise

A shaft or room that has been excavated from the ground upward

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Rhenium

A heavy, dense and rare, silvery-white precious metal. Symbol, Re

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Radon

The heaviest radioactive gas element, symbol, Rn

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Rutile

A tetragonal mineral and ore of titanium, TiO2

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Ruthenium

A silvery white, hexagonal platinum group metal. Symbol, Ru

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Runoff

Water that drains from a flood plain, or higher structure into streams, rivers, or other channels

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Ruby

The red, gemstone variety of corundum Al2O3. Sapphires represent the blue variety of the same substance