often spread
over the substance of the stone in scabs or "splotches," which rather
favours imitation, and, where this unevenness occurs, it may be
necessary to cut or divide the stone, or so to arrange the form of it
that the finished stone shall be equally blue throughout.

In some cases, however, the sapphire may owe its beauty to the presence
of two, three or more colours in separate strata appearing in one stone;
such as a portion being a green-blue, another a cornflower blue, another
perfectly colourless, another a pale sky blue, another yellow, each
perfectly distinct, the stone being cut so as to show each colour in its
full perfection.

This stone, the sapphire, is hardness No. 9 (see "Hardness" table), and
therefore ranks next to the diamond, which makes it a matter of great
difficulty to obtain an imitation which is of the same specific gravity
and of the same degree of hardness, though this has been done. Such
stones are purchasable, but though sold as imitations at comparatively
low price, and the buyer may consider them just as good as the real gem,
to the experienced eye they are readily detectable.

By heating a sapphire its blue colour slowly fades, to complete
transparency in many cases, or at any rate to so pale a tint as to pass
for a transparent stone. Valuable as is the sapphire, the diamond is
more so, and it follows that if one of these clear or "cleared"
sapphires is cut in the "rose" or "brilliant" form--which forms are
reserved almost exclusively for the diamond--such a stone would pass
very well as a diamond, and many so cut are sold by unscrupulous people
as the more valuable stone, which fraud an expert would, of course,
detect.

Sapphires are mentioned by Pliny, and figure largely in the ancient
history of China, Egypt, Rome, etc. The Greeks dedicated the sapphire
specially to Jupiter, and many of the stones were cut to represent the
god; it also figured as one of the chief stones worn by the Jewish High
Priest on the breast-plate. Some stones have curious rays of variegated
colour, due to their crystalline formation, taking the shape of a star;
these are called "asteriated," or "cat's eye" sapphires. Others have
curious flashes of light, technically called a "play" of light (as
described in Chapter VI. on "Colour"), together with a curious blue
opalescence; these are the "girasol." Another interesting variety of
this blue sapphire is one known as "chatoyant"; this has a rapidly
changi