Transparent variety of the silica mineral quartz that is valued for its clarity and total lack of colour or flaws.
Rock crystal formerly was used extensively as a gemstone, but it has been replaced by glass and plastic; rhinestones originally were quartz pebbles found in the Rhine River. The optical properties of rock crystal led to its use in lenses and prisms; its piezoelectric properties (see piezoelectricity ) are used to control the oscillation of electrical circuits.