Gems Notes and Extracts Augusto Castellani, Mrs. John Brogden 1871

PLASMA. 181%LXXXI.PLASMA.UNDER this name many people understand two differentstones, thinking that the ancients confounded themtogether and called them prasius.But I think that the prasina, or prasius of antiquity,isnot the stone which, with more propriety, we nowcall plasma ; moreover, it appears to me they ought tobe accurately distinguished.I shall speak here of the plasma, and afterwards ofthe prasina, showing how this is not a variety of theplasma, and the ancients did not confound them, as itis said, one with the other, but that they indicated theplasma by a different name from the prasius, andperhaps by that of molochites.The plasmais a semi-transparent agate, colouredgreen by some metallic oxide, probably copper ornickel; although often in the purest quality it approachesthe colour of a fine emerald, yet it neverpossesses its brightne-s, is never pure, but alwaysmarked with little black and yellow spots.Like the calcedony, it scratches glass deeply, and itsisspecific gravity from 2'58 to 2'66.In colour it is dark olive, resinous, and semi-transparent.The extreme fineness of its texture renders itsuitable for engraving.It is found in the East and in the Black Forest.As we do not know exactly what name the ancients