Chalcedony is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of silicon dioxide, their full chemical compound being SiO2.
Chalcedony is a compact form of silica, composed of microscopic quartz crystals. It is softer than quartz and denser than opal. Its appearance may range from transparent to translucent to opaque. The main types are chalcedony, which is uniformly colored, and agate, which has curved bands or zones of varying color.
Varieties include the red to reddish brown, translucent carnelian, the red, opaque jasper, the bright-green chrysophrase, moss agate with dark, branchlike patterns, and heliotrope, green with red spots. Flint and chert are impure forms of chalcedony.
Chalcedony occurs in some fossils, such as petrified wood. It forms in cavities in rocks of different types, especially lavas, and develops at relatively low temperatures.
Visible examples would be the agate in Rio Grande de Sul (Brazil), carnelian in Uruguay and California, chrysoprase in Queensland (Australia), and flint in the chalk of the white cliffs of Dover (England).
The name chalcedony probably comes from Calcedon or Chalcedon, an ancient port on the Sea of Marmara, in Asia Minor. The Greek khalkedon and Latin charcedonia do not appear, at least from the description of Pliny the Elder, to be the same mineral as the modern chalcedony. Its abundance and durability has made it one of the earliest materials used by man to make pointed tools, knives, and even food containers.
Chalcedony has been used since time immemorial as gems, because of their color, hardness, and ability to take a good polish, and as precious materials for the production of ornaments or small sculptures.