present,
as well as lime and iron. But it is the presence of sulphur in two
forms of combination, namely as sulphide and as sulphate, which
distinguishes the blue pigment, obtained by the treatment of
lapis-lazuli, from all other blue compounds. This pigment has been very
successfully imitated, not only as to colour but as to chemical
constituents, by chemical art. It should be added here that the minute
brass yellow specks, or spangles which are commonly seen in lapis, are
iron pyrites. Lapis was the sapphire of the ancients.
Lapis-lazuli
occurs in Transylvania, Siberia, Tartary, Persia, China, Thibet and
Brazil. The richly coloured varieties are used for beads and for mosaic
work and inlays in bijouterie, vases, furniture, and even in the
internal decoration of buildings.
The
hardness of lapis-lazuli lies between 5 and 5.5 '. Its specific gravity
is about 2.4. By moderate heating the blue colour of this mineral is
generally unaffected, though in some cases it may actually be deepened.
Carbonic acid does not affect it, nor does a cold weak solution of
alum. Strong acids decompose it, the colour disappearing and
sulphuretted hydrogen being given off.
The
blue mineral called lazulite, although it sometimes presents an
appearance slightly resembling that of lapis-lazuli, is not connected
with the latter species by chemical constitution, for it is a
phosphate, not a silicate (see page 72, under turquoise).
A
thin section of lapis-lazuli constitutes a most beautiful microscopic
slide. The perfect transparency and superb colour of the blue portions
are characteristic. Until thus seen by transmitted light it would not
be imagined that a mineral which appears by reflected light to be
almost opaque could allow the passage of so much light through it.
Iolite,
Called
also dichroite and saphir d'eau, and known to mineralogists as
cordierite, is a beautiful and curious stone, remarkable for its
