Chalcedony  is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz  .Quartz has a trigonal crystal structure. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide). Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black . Commercially

most valuable colours are blue , purple and green .

The name chalcedony comes from  khalkedon  which was small port town during the Roman period in history  . Today it is Kadıköy in Istanbul . At that time , Romans were mining Chalcedony near Eskisehir and exporting via Khalkedon port . For this reason , stone took this name . Today Turkey has several chalcedony and opal deposits in different scales .

Physical Properties of Chalcedony

Lustre: Waxy, Dull

Diaphaneity (Transparency): Translucent

Comment: vitreous when polished

Streak: White

Hardness (Mohs): 6½ - 7

Tenacity: Brittle

Fracture: Conchoidal

Density (measured): 2.6 g/cm3 

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