NDER the name of Callais or Callainat Pliny
describes a greenish gem-stone, which has generally been regarded as
our modern Turquoise. This identification, it is true, is open to some
doubt, but it is, nevertheless, the custom of many mineralogists, to
designate the Turquoise in scientific language by the name of Callaite. In
popular phraseology, however, the beautiful stone is invariably called
Turquoise. Thomas Nicols, in his " Lapidary," says, " The Turquoise is
a hard gem, of no transparency, yet full of beauty : its colour is
sky-blue, out of a green, in which may be imagined a little milkish
infusion. A clear sky, free from all clouds, will most excellently
discover the beauty of a true Turquoise." Its exquisite colour, which
loses nothing by candle-light, is no doubt owing to the presence of a
certain quantity of phosphate of copper. Those specimens of the
Turquoise which retain their colour perpetually,, are said to belong to
the "Old Rock," and are very scarce ; while those that lose their
colour, or become green by exposure, are ascribed to the " New Rock."
Turquoise does not occur crystallised, but is found only in a compact
form, having no cleavage, but possessing a conchoidal fracture. It is
infusible before the blow-pipe, but is readily affected by acids.
Chemically it is a phosphate of alumina, in a hydrated condition ; and
its composition has been investigated with great care by Prof. A. H.