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284 NATURAL HISTORY OF GEMS.
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and
some also exhibited fine Onyx layers. He further mentions that a quarry
of Sards and Calcedonies was then worked near Guzerat.* When Garcias ab
Horto wrote, the Sard and the loadstone were commonly exported from
Cambaya, and sold by the manus, or weight of 2G pounds.
Mohammed
Ben Mansur divides the Carnelian into seven kinds, viz. : the
liver-coloured, rose-red, yellow, white, black, blue, bicoloured
(evidently including certain Agates under this denomination). He adds,
" Although a hard stone, it is commonly used for the engraving of
signets upon." It was then found at Senaa and Aden in Yemen ; on the
confines of India and Burn (i. e. the Byzantine Empire), and in the vicinity of Basra.
Amongst
the other recipes for falsifying gems, alluded to by Pliny, one was "
how the Sardonyx may be made out of a Sard." He seems here to have in
view the artifice by which the former gem (and literally Pliny's own
definition of it, " a white layer upon Sard," " candor in Sarda ") was
imitated by placing a Sard, first coated with carbonate of soda, upon a
red-hot iron. This process converted the surface into an opaque white
layer of any depth required (depending upon the length of the
calcination), which forms a good relief to the intaglio cut through it
into the transparent ground beneath. Doubtless this effect of fire upon
the Sard was discovered by accident;·)· and the method did not come
into general use till a late period
* For the present state of these mines, see p. 228.
t
The ancient practice of committing to his pyre his choicest ornaments
with the corpse, has supplied numerous, and to collectors lamentable
examples of the effect of fire and soda (derived from the wood-ashes)
upon the constitution of tho several gems. I have quoted Cynthia's
Beryl thus sacrificed to her manes. Sards, the backs and edges of which
had been protected from the action of tho soda by their metal casings,
are perpetually to be met with, having their surfaces to a certain
depth calcined into a complete calcedony.
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