COMMON CALCEDONY. *The following are its crystallizations: Supposititious Crystals. 1. Single three-sided pyramid. 2. Double three-sided pyramid. 3. Six-sided pyramid. 4. Six-sided pyramid, sometimes acuminated with three planes. 5. Rhomb. True Crystals. 1. Cube, in which the sides are either drusy or gra nulated, with a glimmering lustre; or smooth, with a splendent lustre. They occur aggregated in druses; It occurs also in extraneous external shapes, in the form of ammonites, turbinites, echinites, madreporites, and of petrified wood. Internally it is dull; the splintery varieties exhibit a faint degree of lustre. The fracture is even ; it sometimes passes into splin tery, sometimes into impertect and ilat conchoidtd. Che reniform varieties exhibit a very delicate fibrous frac ture. The fragments are angular, and pretty sharp edged. It occurs in lamellar distinct concretions, varying in thickness, and which are sometimes reniformly curved,_ sometimes globular and concentrically curved; and very rarely occurs in prismatic concretions, which are thin, and composed of coarse, long, angulo-granular concre tions. It is generally semitransparent; but the black and white varieties are only transluceut. It