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quartz
Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. Its crystal structure is a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2.
There are many different varieties of quartz, several of which are semi-precious gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Europe and the Middle East.
==Etymology==
The word "quartz" is derived from the German word "Quarz" and its Middle High German ancestor "twarc", which probably originated in Slavic, cf. Czech ''tvrdý'' ("hard"), Polish ''twardy'' ("hard"), Serbian and Croatian ''tvrd'' ("hard").
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. Today, the term ''rock crystal'' is sometimes used as an alternative name for the purest form of quartz.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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