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PRECIOUS STONES.
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Zircon,
besides its use as a gem, is now of great value as a source of the
oxide Zirconia; and in North Carolina, in the United States, it is
extensively mined, as well as in other parts.
Use
is made of the effect of heat in discharging the colour of dark Zircon
to produce colourless specimens, and such have been substituted for
Diamond, which they much resemble, on account of their adamantine
lustre, though they lack the fire of the Diamond. The great density of
Zircon is of much help in its identification. The Garnets are
distinguished by their single refraction, and so is Diamond, the latter
also showing more " fire." Of the doubly refracting minerals, Corundum
is the only one likely to be confused with Zircon that approaches it in
density, and the Oriental Hyacinth, as it is called, is strongly
dichroic.
397. Topaz.
What
we know now as Topaz was not the Topazius of Pliny, which seems rather
to have been our Olivine; it may have been included in part in Pliny's
term Chrysolithus.
Topaz occurs colourless and more frequently of a straw or amber colour, or pale green or blue; more rarely pink.
The
lustre is vitreous and especially bright on the prism faces; the
cleavage plane shows a pearly lustre. The mineral in the kinds used as
gems is transparent, though less clear forms are also found. It is
doubly refracting, but the indices of refraction do not differ greatly
from one another, and they are not of high value ; nor is there much
difference in the indices for differently coloured rays. Thus the
greatest and least values for the B line in the red part
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