214 QUARTZ FAMILY. Constituent Parts. Silica, . . 96.75 Alumina, - 0.25 Iron, - - 0 50 Loss, - - . 2 50 t . 100 Klap. Beit. b. iv. s. 326. \ Geognoslic and Geographic Situations. Its geognostic situation is not known : it is associated with common calcedony; and some mineralogists, as Steffens, conjecture that it occurs in beds. Most of the specimens in collections have been found among the ruins of Home. It occurs also at Prussa, at the foot of Mount Olympus, in Asia Minor, where it is associated with the green calcedony of Klaproth, which, however, is only a variety of plasma. • Use. It was considered by the Romans as a gem, and was cut into ornaments; and frequently figures were engraved upon it. Observations. 1. It is distinguished from Heliotrope by colour, infe rior lustre, and weight, and also by its greater transparen cy: it is distinguished from'fihrysoprasc by colour, greater lustre, perfect conchoidal fracture, and greater weighty and its fracture distinguishes it from Common Calcedony- 2. It has been known for several centuries in Italy un der the name Plasma, but was first introduced into the system by Werner. 3. It