Chalcedonius

Chalcedonius is a Latin adjective meaning "of chalcedony" and is used in mineralogy and related fields to describe rocks, minerals, or textures that resemble chalcedony or are composed of chalcedony. Chalcedony, the primary reference, is a cryptocrystalline form of silica (SiO2) characterized by microcrystalline quartz grains intergrown to produce a smooth, waxy to vitreous luster. It occurs in masses that are translucent to opaque and usually ranges from white to gray, though colored varieties exist, including agate, carnelian, onyx, and jasper when coloration and zoning are present.

The name chalcedony derives from Latin chalcedonius, ultimately from Greek khalkedon, referencing the ancient city of Chalcedon, a source historically associated with gemstones. Geologically, chalcedony forms as silica-saturated fluids fill cavities in volcanic rocks and as nodules or veins in sedimentary rocks; it can also occur as replacements during diagenesis. In hand specimen, it is hard (Mohs 6–7), has a conchoidal fracture, and can be carved or faceted for use as gemstones, cabochons, and beads.

In modern mineralogy, chalcedony is a broad term encompassing the cryptocrystalline varieties; chalcedonius, when used as an attributive adjective, largely indicates a relation to chalcedony but is not a separate mineral species. The term chalcedony is preferred in contemporary usage; chalcedonius appears chiefly in older literature and in Latin descriptions.

See also: Chalcedon, Council of Chalcedon.