However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum. The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. Well-formed crystals typically form as a druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples. In nature quartz crystals are often twinned (with twin right-handed and left-handed quartz crystals), distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive. 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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