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144 NATURAL HISTORY OF GEMS.
parte,
nec praeter margines quidquam auro amplectente." The extremely globose
form given to all antique Plasmas and Amethysts strikingly corroborates
this observation; showing that they necessarily wero intended for
setting without a back.
The
conclusion to be derived from all the foregoing details is undoubtedly
this, that the Jaspis of the ancient« was our Calcedony (silica
combined with alumina)—in its primary sense signifying the variety
coloured green by nickel, now called Plasma, but in after times
embracing the blue, the purple, the yellow, the whitey-brown shades of
the same substance ; in a word, every colour except the blood-red, which gave its name and honour to the Sard. This decision is furthermore confirmed by the authority of Ben Mansur, dividing his Jasheb (the original of the Greek Iaspis) into
five kinds : the White clear, the White-yellow, the Green-black, the
Black-transparent, the Dust-coloured : or in modern parlance, the White
Carnelian, the Yellow and Olive Sard, the Brown Sard (ßardoine), and the light Brown.
The modern Jasper, as distinguished from the modern Agate
only in its being more opaque and containing more iron in its
composition, was certainly regarded by the ancients as a variety of
the Achates. Of this we shall now proceed to consider the kinds that
were chiefly employed by the engravers of antiquity.
1.
First in the class stands the Black Jasper, an extremely fine,
close-grained substance, perfectly opaque, and thus distinguishable
from the Black Agate (to which the ancients were equally partial),
which is translucent and reddish-brown by transmitted light. This stone
takes a high polish, the good preservation of which declares the
uncommon density of its particles. It has been employed by the Greeks
as the material for some of their finest
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