The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. Quartz belongs to the trigonal crystal system at room temperature, and to the hexagonal crystal system above 573 ☌ (846 K 1,063 ☏). Today, the term rock crystal is sometimes used as an alternative name for transparent coarsely crystalline quartz. The Ancient Greeks referred to quartz as κρύσταλλος ( krustallos) derived from the Ancient Greek κρύος ( kruos) meaning "icy cold", because some philosophers (including Theophrastus) apparently believed the mineral to be a form of supercooled ice. The word "quartz" is derived from the German word "Quarz", which had the same form in the first half of the 14th century in Middle High German and in East Central German and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý ("hard").
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