The
Western turquoises are also osseous substances, generally the teeth of
animals, but in a natural state, and not petrified. They are coloured
by oxide of copper, and are more likely than the others to discolour
under the influence of the air.
They lose their colour in distilled water, and dissolve under the action of acids, especially of aquafortis.
It
is commonly believed that in certain cases every turquoise loses its
fine colour. In the middle ages it was asserted that the turquoise grew
pale on the finger of a sickly person, but that it regained its colour
on the hand of a perfectly healthy person. Others asserted that its
colour varied with the hours of
