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PLASMA. 181
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LXXXI.
PLASMA.
under this name many people understand two different stones, thinking that the ancients confounded them together and called them prasius.
But I think that the prasina, or prasius of
antiquity, is not the stone which, with more propriety, we now call
plasma ; moreover, it appears to me they ought to be accurately
distinguished.
I
shall speak here of the plasma, and afterwards of the prasina, showing
how this is not a variety of the plasma, and the ancients did not
confound them, as it is said, one with the other, but that they
indicated the plasma by a different name from the prasius, and perhaps by that of molochites.
The
plasma is a semi-transparent agate, coloured green by some metallic
oxide, probably copper or nickel ; although often in the purest quality
it approaches the colour of a fine emerald, yet it never possesses its
brightness, is never pure, but always marked with little black and
yellow spots.
Like the calcedony, it scratches glass deeply, and its specific gravity is from 2-58 to 2-66.
In
colour it is dark olive, resinous, and semi-transparent. The extreme
fineness of its texture renders it suitable for engraving.
It is found in the East and in the Black Forest. As we do not know exactly what name the ancients
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