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NATURAL HISTORY OF GEMS.
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meant by Ostracias is favoured by a passage
in the same hard-to-be-understood author, where he describes the
polishing of Roek-Crystal by means of " fornacis fragmine, micas,"
ground to powder, and spread upon a leaden plate,—
" Haec etenim plumbum conjunctio reddit acutum, Et suum rursus habent lateris fragmenta vigorem."
Yet
it seems hardly possible that any merely vitrified matter should be
sufficiently hard to be thus employed as a substitute for emery, even
though " fornacis fragmen " means the Ostracias of the copper-furnace.
Again,
if we suppose the name Ostracitis to be derived from the resemblance to
a shell, it is a singular coincidence that fossil shells, especially
echini and small ammonites, are very plentful, coniposed entirely of
Iron Pyrites, forming beautiful golden objects which must have often
excited the admiration of the ancient miners.*
From
its use to the lapidary the Marcasite was an important article with
the Persians. Ben Mansur devotes a separate chapter to it, dividing the
kinds, according to their colour, into gold, silver, iron, and copper
Marcasites. Marcasites, when facetted, have a true adamantine lustre
(though perfectly opaque), which they preserve without tarnishing. In
the last century they were much used in jewelry as a cheap substitute
for Diamonds. Pyrites, from their supposed fiery nature (they were the
equivalent for flint in the tinder-box of the Greeks and Italians, and
in the early wheel-lock guns of the latter) were esteemed powerful
agents in the reducing of tumours ; being roasted, ground. and made
into a plaster. Dioscorides and Galen give recipes for their employment.
* "Which Pliny actually records of the latter fossil. "Amnion's Horn, reckoned among the most holy gems of Ethiopia, is of a golden colour, and presents the image of a ram's horn : it is pretended that it procures prophetic dreams " (60).
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