Chalcedony is a
cryptocrystalline form of silica, composed of very fine
intergrowths of the minerals quartz and moganite. These are both
silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure is SiO2. Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white
to gray, grayish-blue or a shade of brown ranging from pale to
nearly black

In all ages chalcedony has been the stone most used by the gem
engraver, and many coloured varieties are still cut and polished
as ornamental stones. Chalcedonic pseudomorphs after other
minerals often give rise to very interesting specimens. Hollow
nodules of chalcedony containing water and an air bubble that is
visible through the semitransparent wall have been found.