A form of fine-grained quartz that is nearly transparent or has a milky translucence; it fractures conchoidally. (noun)
Examples of word chalcedony
There is a kind of onyx called a chalcedony, which hath the same qualities,  avails much against fantastic illusions which proceed from melancholy, preserves the vigour and good estate of the whole body.
As to the second, of which the name does not occur in treatises on gems, it may be identified with chalcedony, which is described as a sort of agate of an opaque quality, dimmed with clouds and flashing fires in the shadows.
Of the seven necklaces pictured here, all purchased recently, three are varieties of chalcedony, which is itself a type of microcrystalline quartz.
They were often made of semiprecious stones such as chalcedony, rock crystal, and lapis lazuli.
The many semi-precious stones which have a quartz basis (such as the varieties of waxy or cryptocrystalline chalcedony which is largely quartz in a very minutely crystalline condition) are often even tougher than the clear crystallized quartz.