See our: rock crystal jewelry.Natural quartz that is crystalline, usually colourless (or nearly so), and transparent (or nearly so). Objects of rock crystal were carved in medieval Egypt,Iraq, and Persia, and were highly prized during the Renaissance. In modern times it is less often faceted, as lead glass is more readily available, but some pieces are carved as acameo or intaglio or as a seal. Rock crystal is also used for beads,bracelets, earrings, etc., faceted or merely polished.It is distinguishable from glass by its greater hardness, coldness, and double refraction. It is sometimes used to simulate colourless gemstones (e.g. stonescalled 'diamond' with various geographical prefixes), but is distinguishable by its lack of fire. It is generally referred to merely as 'crystal', although that term is also now used for glass madewith lead oxide and having a high brilliance like rock crystal. The German term is bergcristal, the French is cristal de roche.