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Brief Description: In the ancient times (Theophrastus) and until the mid-17th century, the clear colourless quartz was called crystal = ice, from the Greek words kryos (cold) and stello (send). Varieties of quartz are: 1) Amethyst: from the negative ?a? and the Greek word methysko (get drunk) because in ancient times it was believed to protect from drunkenness. 2) Smoky Quartz: from the dark smoky-brown colour and 3) Aventurine: due to its typical radiant lustre. It has the chemical formula SiO2 and it belongs to the category Tectosilicates (SiO2 group). Its lustre is vitreous in microcrystalline varieties and dull or sebaceous in cryptocrystalline varieties. It is white-coloured or colourless. It often contains very small inclusions that give it colours, like milky, violet, brown, red, yellow, green, black etc. Its specific gravity is almost 2.65, its hardness 7 and its fusibility 7 as well. It has prismatic form or rhombohedral that looks like pyramidal. The prismatic sides usually have horizontal stripes. It occurs in crystals and solid masses. It can also be found in geodes. Quartz is a very common and widespread mineral. It can be found in several igneous and transformed rocks and because it is tough against mechanical and chemical decomposition, it occurs in many sedimentary rocks and in sand as well. It has various uses. Its various versions are used as precious and semiprecious stones (amethyst, citrine, agate). The quartz sand is used as abrasive, as well as in glass and silicon bricks industry. In powder, it is used in porcelain and colour industry and as rock (sandstone, quartzite) for construction purposes. It is used in scientific optical instruments (lenses, prisms) and because of its powerful piezoelectric properties, it is used in the manufacture of digital watches and elsewhere. Some macrocrystalline varieties of quartz are the following: 1) Rock crystal: colourless and clear variety of quartz. It usually occurs in monocrystals. 2) Amethyst: violet variety of quartz. It usually occurs in earthen. Its colour is distributed in small sums B or BPO4 or according to others Fe3+. 3) Smoky Quartz: dark-coloured in the colour of soot, brown, grey to black variety of quartz. The back variety is called morion. Transparent to semitransparent, opaque. It usually occurs in crystals. The colour is attributed to loose ions Si that are formed when quartz is exposed to radiation of radioactive minerals. 4) Aventurine: thin orientated leaflets of chromium mica (green) and hematite (red) that are enclosed and reflect the light giving it a radiant lustre. 
Bibliography: Ευρετήριο ορυκτών ΑΠΘ στην ιστοσελίδα http://www.geo.auth.gr/106/az_gr.htm Τσιραμπίδης Α. Ο ορυκτός πλούτος της Ελλάδας. Εκδόσεις Γιαχούδη Θεσσαλονίκη, 2005, σελ. 391. Από την ελεύθερη ηλεκτρονική εγκυκλοπαίδεια Βικιπαιδεία στην ιστοσελίδα http://en.wikipedia.org/ Berry, L.G., Mason, B. and Dietrich, R.V. (1983). Mineralogy. W. H. Freeman and Company, San Francisco, 561 pp. Σαπουντζής, Η. και Χριστοφίδης, Γ. (1985). Ορυκτοδιαγνωστική. University Studio Press, Θεσσαλονίκη, 241 σ.