n.
or calcedony
Very fine-grained variety of the silica mineral quartz .
A form of chert , it occurs in a great variety of colours, usually bluish white, gray, yellow, or brown. Other physical properties are those of quartz. For centuries, chalcedony has been the stone most used by gem engravers, and many varieties are still cut and polished as ornamental stones. See also agate ; carnelian ; onyx .