276	OSTRACIAS.
	OSTEACIAS, or OSTRACITIS : Marcasite ? Pyrites ? Thus briefly noticed by Pliny (65) :—" Like a potsherd (tes-tacea), of a harder quality than the Ceramitis ; ' resembling the Agate, except that the Agate becomes smooth and fatty by polish­ing. The harder kind possesses such power that gems may be engraved with its fragments. The Ostracitis derives its name from its similarity to a shell (or oyster)." Two distinct sub­stances are here described ; the confusion arising from the cir­cumstance that οστρακον means both a potsherd and an oyster-shell. Hence the Ostracias was some mineral capable of being used in gem-engraving, resembling burnt clay in colour, rough, and incapable of polish : the Ostracitis merely a fossil shell. What the Ostracias really was may be conjectured from the first notice of it (56) :—" The Cadmitis is the same as what is called Ostracitis, except that the last is sometimes surrounded by azure bubbles (bullae)." Now Cadmia is supposed to mean Zinc-ore, or Calamine-stone.2 Hence it would appear that the Ostracias of the Greek gem-engravers was the Marcasite, or Iron Pyrites, which, fractured, much resembles Calamine-stone ; when roasted, turns to a rusty red (testacea), and which has been used from time immemorial in Persia for polishing the harder gems. M. Ben Mansur states, " the Laal (Spinel) takes a polish with difficulty, and for a long time they were unable to polish it, until at last they brought it about by means of the Gold Marcasite called Ebrendshe." De Laet, in 1647, states that the Spinel and Balais could only be polished by means of the Pyrites. Modern lapidaries use vitriol for the same purpose, the principle of both processes being iden­tical, vitriol being merely the extract of the Iron Pyrites.