has
the unfortunate peculiarity of changing its hue in the most rapid and
mysterious manner; a fine blue stone will turn on the lapidary's wheel
to a sickly green or whitish tint; other specimens retain their colour
for some weeks, breaking out afterwards in an eruption of white specks,
which gradually overspread the whole surface, whilst others again
begin to whiten or to become green first round the edge; some
specimens regain their colour by being soaked in water or weak uric
acid, but lose it again as the stone becomes dry. In certain rare
instances, however, the colour has remained unchanged for many years ;
but, as a rule, it would be well never to give a large price for any
turquoise from this mine. They may be readily distinguished from the
real turquoise de vieille roche by the stratum (in most cases apparent at the back) being of a pale yellowish-red colour, instead of dark brown.
The
Persian turquoise is also subject to change of colour, although in
nothing like the proportion of the variety mentioned above; if not
brought into contact with acids, musk, camphor, or other scents, it
retains its hue for many years, turning at last to a green or a
white,—although ancient cameos and intaglios are extant which have
retained their colour until the present day. Many persons still hold to
the belief that this gem by its changes indicates the state of health
of the wearer, and perhaps the fact that turquoises do vary their
colour in the most unaccountable manner, may have something to do with
this old superstition.
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