lor of some
genuine but inexpensive gem material.

For example, large quantities of somewhat porous chalcedony from Brazil
are stained and sold in imitation of natural agate or sard or other
stones. In many cases the staining is superficial, so that the stone has
to be shaped before it is stained, then stained and polished.

Large quantities of slightly crackled quartz are stained to resemble
lapis lazuli, and sold, usually with the title "Swiss Lapis." A file
test will reveal the character of this imitation, as it is harder than a
file, while true lapis is softer. The color too is never of so fine a
blue as that of fine lapis. It has a Prussian blue effect.

Turquoises of inferior color are also sometimes stained to improve
them. A better product is made artificially.

Opals are sometimes impregnated with organic matter, which is then
charred, perhaps with sulphuric acid, thus giving them somewhat the
appearance of black opal.

Opals are also imitated by adding oxide of tin to glass, thus imparting
a slight milkiness to it. The imitation is then shaped from this glass
by molding, and the back of the cabochon is given an irregular surface,
which may be set over tinsel to give the effect of "fire."

Pale stones are frequently mounted over foil, or in enameled or stained
settings and thus their color is seemingly improved.

Diamonds of poor color are occasionally "painted"; often the back of the
brilliant is treated with a violet dyestuff, which even in so small an
amount that it is difficult to detect, will neutralize the yellow of the
stone and make it appear to be of a fine blue-white color. The
"painting" is, of course, not permanent, so that such treatment of a
diamond with a view to selling it is fraudulent. The painted stone may
be detected by washing it with alcohol, when the dye will be removed and
the off-color will become apparent. If the stone is unset one can see
with a lens a wavery metallic appearance on the surfaces that have been
"painted." This effect is due to the action of the very thin film of dye
upon the light that falls upon it.

Besides the staining of genuine materials, they are sometimes altered in
color by heat treatment, and this topic will be discussed in the next
lesson.

LESSON XXVI

ALTERATION OF THE COLOR OF PRECIOUS STONES

Many gem minerals change color when more or less strongly heated.
Extreme heat whitens many colored materials completely.

"PINKED TOPAZ." John