Chalcedony is one of the cryptocrystalline varieties of the mineral quartz, having a waxy luster. Chalcedony may be semitransparent or translucent and is usually white to gray, grayish-blue or some shade of brown, sometimes nearly black. Other shades have been given different names. A clear red chalcedony is known as carnelian or sard; a green variety colored by nickel oxide is called chrysoprase. Prase is a dull green and onyx is black and white banded. Plasma is a bright to emerald-green chalcedony that is sometimes found with small spots of jasper resembling blood drops; it has been referred to as blood stone or heliotrope. Flint is also a variety of chalcedony.
Chalcedony cameo of Titus head, 2nd Century ADPeople living along the Central Asian trade routes used various forms of chalcedony, including carnelian, to carve intaglios, ring bezels (the upper faceted portion of a gem projecting from the ring setting), and beads that show strong Graeco-Roman influence. Fine examples of first century objects made from chalcedony, possibly Kushan, were found in recent years at Tillya-tepe in north-western Afghanistan. Hot wax would not stick to it so it was often used to make seal impressions.
The term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English usually spelled Chalcedon, today the Kadiköy district of Istanbul.
Chalcedony is more soluble than quartz under low-temperature conditions. It is, however, crystallographically identical to quartz. A mineral with the exact same crystallographic and chemical properties should have the same bulk thermodynamic properties (properties which are used to predict solubility). But chalcedony is extremely finely grained (cryptocrystalline), and has a very high surface area to volume ratio. It is believed that this property of chalcedony is responsible for its higher solubility.