Chalcedony

Chalcedony owes its name to the ancient city of Chalcedon, now called Kadıköy, a district of the city of Istanbul. Depending on the context, the term chalcedony has different meanings. It can be understood as a general term that indicates the varieties of quartz that are made of microscopic or submicroscopic crystals, the so-called microcrystalline varieties of quartz. These include agate, jasper, chrysoprase, carnelian and onyx.
The term chalcedony also indicates the mineral itself, a fibrous and cryptocrystalline variety of quartz. It is a secondary origin mineral deriving from the alteration of the silicates of the encasing rocks. They form the filling of more or less large cavities, within igneous rocks especially in the basic ones (geodes); these cavities are produced by the expansion of gases and magma vapors. Zoned structure are frequent in chalcedony and mainly clear in varieties that have color differences in individual areas such as agate and onyx. When pure, the chalcedony is whitish-bluish gray in color; however, the coloring is very variable as a consequence of impurities. The chalcedony is translucent to almost opaque.
Widely used and appreciated in the ancient world, it was thought to come exclusively from Egypt and Arabia. It is mainly mined in Uruguay and southern Brazil. The purest masses come from Ireland, the Faroe Islands, Syria, the East Indies, Greenland and the island of Sri Lanka.