Brief Description: Its name originates from the ancient Greek city Chalcedon in Bithynia of Asia Minor, opposite Byzantium. It has the chemical formula SiO2 and belongs to the category Tectosilicates (SiO2 group). It constitutes a fine-grained (cryptocrystalline) variety of quartz and a form of chert. Its lustre is sebaceous or vitreous, with colour ranging from white, grey, grey-green, grey-blue, yellowish, brown, deep red or green. Its specific gravity is almost 2.6. Its hardness is 6 and its fusibility 7 as well. It is semitransparent to opaque. With the term chalcedony, we refer to solid varieties of silica that consist of tiny, fibrous crystals. Chalcedony occurs in botryose, stalactite-like aggregates, in geodes, as substitute of fossils and other minerals. It is formed by hydrous solutions in average to low temperatures. It is used as an ornamental stone. Some cryptocrystalline varieties of Chalcedony are: 1) Agate: The name originates from the river Achates (Dirillo in our days) of Sicily (source: Theophrastus, Plinius). It consists of two consecutive layers of chalcedony with different colour. The coloured layers are often very thin, parallel, curved and concentric. 2) Onyx: The name originates from its comparison to the human nail (Theophrastus, Plinius). Theophrastus also mentions it as onichion. It is agate with consecutive dark-coloured and light-coloured layers. Sardonyx is onyx, where the dark layers are red or brown. 3) Jasper: It is mentioned as iaspis by Theophrastus. It is opaque, dull, granular, chalcedone with usually dark red, brown-red or yellow-brown colour. Its colour is due to mixtures of hematite and goethite. 4) Heliotrope or bloodstone: The name originates from Greek words hellion (sun) and trepo (change) (source: Plinius) because it was possibly used in glass manufacture for solar observations. It is a green chalcedone with small red spots of jasper that look like blood stains. 5) Chert ? Flint: The name originates from its similarity to a horn (ceras) in colour, fracture and transparency. In the prehistoric era, chert and flint were used to make tools and weapons. Their differentiation is not clear. Usually, chert refers to the cryptocrystalline, solid, opaque and unshaped quartz that has either organic (diatoms, radiolarites) or inorganic origin and occurs in form of extended layers in sedimentary rocks. Flint is a kind of chert with a characteristic conchoidal fracture and it occurs in form of condyls. Lydite (that is mentioned as Lydi by Theophrastus and comes from river Tmolus of Lydia) is a black, clean, solid, variety of chert that is used in order to define the quantity of gold in various alloys. 
Bibliography: Ευρετήριο ορυκτών ΑΠΘ στην ιστοσελίδα http://www.geo.auth.gr/106/az_gr.htm Τσιραμπίδης Α. (2005). Ο ορυκτός πλούτος της Ελλάδας. Εκδόσεις Γιαχούδη Θεσσαλονίκη, σελ. 391. 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.