the latter class are pyrite, which is brass yellow, lapis lazuli, which is blue, and malachite, which is green.
In
a few cases differences of chemical composition are indicated by
differences of color. This is true of garnet, the magnesium - aluminum
varieties of which are ruby red, the calcium-aluminum varieties
brownish red, and the calcium-chromium varieties green. So tourmaline,
when containing an excess of iron, is black; an excess of sodium and
lithium is green or red, and an excess of magnesium is brown.
Usually, however, the coloring matter is foreign to the essential composition of the mineral, and of very small amount.
This
coloring ingredient is in the majority of cases organic matter of some
sort, chiefly hydrocarbons. This has been proved in some cases by
analysis, and in general may be assumed when the color of a stone can
be driven out or changed by heat. The following gems quite certainly
owe their color wholly or in part to organic matter:—smoky quartz,
amethyst, yellow topaz, golden beryl, zircon, rubellite, and amazon
stone. The coloring ingredients of the following are chiefly
inorganic:—ruby, sapphire, spinel, and emerald.
Next
to organic matter metallic oxides are probably the most prevalent
coloring ingredient. These oxides may occur in scales large enough to
be seen with the naked eye, as is true of the hematite in sunstone, or
they may be only visible with the microscope, as the same substance can
be seen coloring jasper and carnelian. More commonly the coloring
matter cannot be discerned as a distinct pigment. Beside oxide of iron
as a coloring ingredient, chromium, copper and nickel oxides occur,
producing in general green colors. Manganese oxide often gives purple
or flesh colors.
By
producing some chemical change it is often possible to alter the color
of a mineral. In the case of minerals colored by hydrocarbons, these
changes may best be produced by heating. In this manner smoky quartz
can be changed in color to yellow, yellow topaz to pink, and brown
carnelian to red.
Amethyst,
hyacinth, and golden beryl lose their color entirely if heated any
length of time, and smoky quartz may also be made colorless by long
continued heat. Some gems change in color on heating, but regain it
again when cooled. Thus pyrope turns darker on heating, but returns to
its normal color on cooling. Ruby becomes colorless, but on cooling
changes through green to its original red.
Some
colors of gems fade or change on exposure to light, a peculiarity which
is of course considered detrimental to their value. In this manner the
blue of turquois may change in time to green, and yellow topaz,
chrysoprase, and rose quartz may lose their color entirely.
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